Coloration Mechanism Research Articles

Self-assembled, disordered structural color from fruit wax bloom.

Many visually guided frugivores have eyes highly adapted for blue sensitivity, which makes it perhaps surprising that blue pigmented fruits are not more common. However, some fruits are blue even though they do not contain blue pigments. We investigate dark pigmented fruits with wax blooms, like blueberries, plums, and juniper cones, and find that a structural color mechanism is responsible for their appearance. The chromatic blue-ultraviolet reflectance arises from the interaction of the randomly arranged nonspherical scatterers with light. We reproduce the structural color in the laboratory by recrystallizing wax bloom, allowing it to self-assemble to produce the blue appearance. We demonstrate that blue fruits and structurally colored fruits are not constrained to those with blue subcuticular structure or pigment. Further, convergent optical properties appear across a wide phylogenetic range despite diverse morphologies. Epicuticular waxes are elements of the future bioengineering toolbox as sustainable and biocompatible, self-assembling, self-cleaning, and self-repairing optical biomaterials.

Black-and-white pelage as visually protective coloration in colobus monkeys

Conspicuous coloration is often seen as the evolutionary consequence of either sexual selection or warning predators visually about prey defences, although not all conspicuous species fit this paradigm. Exceptions include several species of colobus monkeys whose black-and-white coloration, characteristic of larger colobines, has never been explained. Here, using photographs of black-and-white colobus (Colobus vellerosus) taken in the wild, quantitative image analysis, and vision modelling (acuity-corrected feline, chimpanzee, and raptor vision), we provide evidence that their coloration provides distance-dependent camouflage in natural forest environments. At all viewing distances, black-and-white colobus monkeys blend into their environment because of their high chromatic overlap with the background when viewed by ecologically relevant predator vision models which have low visual acuities. Additionally, for chimpanzee and felid vision, there was evidence of edge disruption at longer viewing distances. Our comparative analyses of different species of colobines do not support an association between black-and-white coloration and larger body mass or group size, but this may simply be due to the limited number of species within the family. We reason that black-and-white colobines gain visually protective coloration through background matching against felids, and also benefit through disruptive coloration against felids and chimpanzees but rely on these protective coloration mechanisms less against raptors.Significance statementConspicuous black-and-white coloration in colobus monkeys has long eluded an evolutionary explanation. Our research, employing photographs of colobus monkeys, image analysis, and vision modelling, unveils a novel approach to this coloration. We show that in their natural forest habitats, black-and-white colobus monkeys employ distance-dependent camouflage against ecologically relevant predators with low visual acuity, blending in with the background at increasing viewing distances. In summary, the evolution of black-and-white colobus coloration appears to be related to background matching against felids, with added benefits of disruptive camouflage against felids and chimpanzees, less so to raptors, offering fresh insights into the complex interplay of coloration and survival strategies in mammals.

Influence of structural colour on the photoprotective mechanism in the gametophyte phase of the red alga Chondrus crispus.

Marine life is populated by a huge diversity of organisms with an incredible range of colour. While structural colour mechanisms and functions are usually well studied in marine animal species, there is a huge knowledge gap regarding the marine macroalgae (red, green and brown seaweeds) that have structural coloration and the biological significance of this phenomenon in these photosynthetic organisms. Here we show that structural colour in the gametophyte life history phase of the red alga Chondrus crispus plays an important role as a photoprotective mechanism in synergy with the other pigments present. In particular, we have demonstrated that blue structural coloration attenuates the more energetic light while simultaneously favouring green and red light harvesting through the external antennae (phycobilisomes) which possess an intensity-dependent photoprotection mechanism. These insights into the relationship between structural colour and photosynthetic light management further our understanding of the mechanisms involved.

The Evolution of Multiple Color Mechanisms Is Correlated with Diversification in Sunbirds (Nectariniidae).

How and why certain groups become speciose is a key question in evolutionary biology. Novel traits that enable diversification by opening new ecological niches are likely important mechanisms. However, ornamental traits can also promote diversification by opening up novel sensory niches and thereby creating novel inter-specific interactions. More specifically, ornamental colors may enable more precise and/or easier species recognition and may act as key innovations by increasing the number of species-specific patterns and promoting diversification. While the influence of coloration on diversification is well-studied, the influence of the mechanisms that produce those colors (e.g., pigmentary, nanostructural) is less so, even though the ontogeny and evolution of these mechanisms differ. We estimated a new phylogenetic tree for 121 sunbird species and combined color data of 106 species with a range of phylogenetic tools to test the hypothesis that the evolution of novel color mechanisms increases diversification in sunbirds, one of the most colorful bird clades. Results suggest that: (1) the evolution of novel color mechanisms expands the visual sensory niche, increasing the number of achievable colors, (2) structural coloration diverges more readily across the body than pigment-based coloration, enabling an increase in color complexity, (3) novel color mechanisms might minimize trade-offs between natural and sexual selection such that color can function both as camouflage and conspicuous signal, and (4) despite structural colors being more colorful and mobile, only melanin-based coloration is positively correlated with net diversification. Together, these findings explain why color distances increase with an increasing number of sympatric species, even though packing of color space predicts otherwise.

Integrative Transcriptomic and Metabolomic Analyses of the Mechanism of Anthocyanin Accumulation and Fruit Coloring in Three Blueberry Varieties of Different Colors

Blueberries are recognized worldwide as one of the most important healthy foods due to their anthocyanins, which have special antioxidant properties. They have become a highly produced and valuable fruit crop. Most blueberry varieties are rich in anthocyanins, which impart a beautiful blue color; however, there are currently several blueberry varieties with different colors worldwide, and these special-colored varieties are the key to analyzing the coloring mechanism of blueberry fruit. Fruit color could be seen as an important nutritional quality trait in terms of marketing. In this study, a combination of transcriptomic and metabolomic analyses was performed on three representative blueberry varieties (‘Pink Popcorn’, ‘Chandler’, and ‘Black Pearl’) with pink, blue, and black fruits, respectively. The metabolomic results showed that the delphinium pigment is the dominant anthocyanin, which is the prerequisite for the formation of fruit color in blueberries. We identified 18 candidate structural genes in the anthocyanin biosynthesis pathway that were significantly up-regulated during three stages of fruit ripening in ‘Black Pearl’ and ‘Chandler’, but these were not found to be significantly expressed in ‘Pink Popcorn’ after combining the transcriptomic analysis results. The non-expression of the VcANS gene may lead to the pink color of the mature fruit of ‘Pink Popcorn’. The phylogenetic tree, heatmap analysis, and WGCNA analysis identified a candidate transcription factor, VcMYBA, which may regulate the differences between black and blue fruits in blueberries by regulating the expression level of multiple structural genes in the anthocyanin biosynthesis pathway. These results provide new insights into the mechanisms of anthocyanin accumulation and coloration in blueberries during fruit ripening and can help support production practices to improve fruit quality characteristics. The key candidate genes that regulate the fruit color differences among different blueberry varieties have the potential to enhance the antioxidant properties and quality characteristics of blueberries through future genomic editing.

Comparative Metabolome and Transcriptome Analyses Reveal the Regulatory Mechanism of Purple Leafstalk Production in Taro (Colocasia esculenta L. Schott).

Taro is a plant in the Araceae family, and its leafstalk possesses significant botanical and culinary value owing to its noteworthy medicinal and nutritional attributes. Leafstalk colour is an essential attribute that significantly influences its desirability and appeal to both breeders and consumers. However, limited information is available about the underlying mechanism responsible for the taro plant's colouration. Thus, the purpose of the current study was to elucidate the information on purple leafstalks in taro through comprehensive metabolome and transcriptome analysis. In total, 187 flavonoids, including 10 anthocyanins, were identified. Among the various compounds analysed, it was observed that the concentrations of five anthocyanins (keracyanin chloride (cyanidin 3-O-rutinoside chloride), cyanidin 3-O-glucoside, tulipanin (delphinidin 3-rutinoside chloride), idaein chloride (cyanidin 3-O-galactoside), and cyanidin chloride) were found to be higher in purple taro leafstalk compared to green taro leafstalk. Furthermore, a total of 3330 differentially expressed genes (DEGs) were identified by transcriptome analysis. Subsequently, the correlation network analysis was performed to investigate the relationship between the expression levels of these differentially expressed genes and the content of anthocyanin. There were 18 DEGs encoding nine enzymes detected as the fundamental structural genes contributing to anthocyanin biosynthesis, along with seven transcription factors (3 MYB and 4 bHLH) that may be promising candidate modulators of the anthocyanin biosynthesis process in purple taro leafstalk. The findings of the current investigation not only provide a comprehensive transcriptional code, but also give information on anthocyanin metabolites as well as beneficial insights into the colour mechanism of purple taro leafstalk.

Deciphering scavenger receptors reveals key regulators in the intestine that function in carotenoid coloration of leopard coral groupers (Plectropomus leopardus)

Carotenoid based body coloration are common features in fish, which depends on the diet derived carotenoids pigments deposition, employing a bunch of carotenoid uptake, absorption and processing related genes. Scavenger receptors are a large family of cell surface receptors with complex structure and diverse functions. However, the SRs genes have been insufficiently explored concerning their role in fish carotenoid coloration. Here, we systemically identified 19 SRs family genes and investigated their expression patterns of in various tissues of P. leopardus. Expression analysis unveiled the diverse involvements of SRs in the intestine of P. leopardus with different body colors and the responses to exogenous carotenoids. Notably, cd36, emerged as a pivotal factor in intestinal functions predominantly localized in the intestinal epithelial and goblet cells. Knockdown of cd36 led to the reduction in skin brightness and carotenoid levels in both intestine and skin, while overexpressing cd36 increased the carotenoids uptake of cells in vitro. Additionally, our investigations revealed that cd36 exerts regulation on genes associated with carotenoid uptake, transport, and processing. To sum up, our results provide a comprehensive view on SRs functions in carotenoid coloration of P. leopardus and will facilitate the understanding on the mechanism of carotenoids coloration of vertebrates.

In-situ synthesis and performance evaluation of novel color coatings for galvanized steel

Traditional hot-dip galvanized coatings, typically single-silver in color, present increasing aesthetic challenges in diverse applications, while prevailing organic coating methods involve complex processes with high energy consumption and pollution emissions. In this study, a novel hot-dip process for in-situ synthesis of color coating on steel was proposed, capable of producing various uniform color coating. The microstructure of color coatings on steel was systematically examined, and an assessment of their corrosion and weather resistance was conducted. The results demonstrate the sequence of typical color variation is gold, purple, blue, cyan, red, green, and grey. By controlling the immersion temperature range, and regulating the final stable color of the oxide film. The coloration mechanism mainly is attributed to the light interference on the Mn oxide film. The cyan and brown coatings mainly consist of the dense rod-like ζ-FeZn13 phases which provide a structural foundation for corrosion resistance. Furthermore, the brown coating exhibited excellent weather resistance.

Combined Widely Targeted Metabolomic, Transcriptomic, and Spatial Metabolomic Analysis Reveals the Potential Mechanism of Coloration and Fruit Quality Formation in Actinidia chinensis cv. Hongyang.

Postharvest kiwifruit (Actinidia chinensis cv. Hongyang) pulp is mainly composed of outer yellow-flesh (LR) and inner red-flesh (HR). However, information about the differences in coloration and fruit quality between these two parts are limited. In this study, widely targeted metabolomic, transcriptomic, and spatial metabolomic analyses were used to reveal the potential mechanism of coloration and fruit quality formation. The results show that a total of 1001 metabolites were identified in Hongyang kiwifruit, and the accumulation of 211 metabolites were significantly higher in the HR than LR, including 69 flavonoids, 53 phenolic acids, and 38 terpenoids. There were no significant differences in the content of citric acid, quinic acid, glucose, fructose, or sucrose between the LR and HR. These results were consistent with the results from the RNA-seq profile and spatial metabolomic analysis. In addition, a total of 23 key candidate genes related to flesh color and fruit quality formation were identified and validated by qRT-PCR analysis. This study provides a theoretical basis for elucidating the underlying mechanism of the formation of kiwifruit flesh color and fruit quality.

Bioinspired multi-dimensional anti-counterfeiting device by combining electrochromism and structural color

Multi-dimensional anti-counterfeiting technologies possess multiple color/pattern variations under different conditions, opening up new avenues in information security. Herein, inspired by the blue-ringed octopus, novel multi-dimensional anti-counterfeiting devices are reported by combining electrochromism and structural color with different coloration mechanisms, that is, by introducing colloidal photonic crystals (PCs) in electrolyte of the electrochromic devices (ECDs). Through actively manipulating voltage or passively changing environment background, the degree of light reflectance and transmittance of PCs can be finely modulated, thus realizing the display and hiding of structural colors of devices. Besides, they also feature distinct color evolution under different viewing angles, enabling more optical information to be carried. Laser-etched patterned devices have also been successfully achieved, showcasing superior triple anti-counterfeiting abilities in terms of optical information. Notably, such inherent patterns can further be innovatively switched to different preset background patterns through the synergy of voltage stimulation and background colors. Based on this, a high-level anti-counterfeiting system has been demonstrated, providing the ability to deliver multiple distinct optical information on-demand. Itis believed that such strategy of combining electrochromic and structural color will pave a new platform for designing advanced multi-dimensional anti-counterfeiting devices.

Coloration differences in three Camellia reticulata Lindl. cultivars: 'Tongzimian', 'Shizitou' and 'Damanao'.

Camellia reticulata Lindl., also known as Yunnan Camellia, is an important ornamental plant in China, especially for its large and stunning flowers. A comprehensive understanding of their coloration mechanisms can aid breeders in developing new cultivars and improving their ornamental value; however, it is still unclear in Yunnan Camellia, especially in mixed-color flowers. In this study, we conducted metabolic and transcriptomic comparison analyses to investigate the coloration differences in three Yunnan Camellia cultivars: C. reticulata 'Shizitou' (SZT), C. reticulata 'Damanao' (MN), and C. reticulata 'Tongzimian' (TZM). Our results revealed that the initial flowering stage may play a critical role in the color change of MN. Metabolome analysis demonstrated that cyanidin was the primary anthocyanin in SZT and MN's red region, while its content was low in TZM and MN's white region. According to the transcriptome analysis, the anthocyanins biosynthesis pathway was reconstructed in Yunnan Camellia, and the low expression of CHS was detected in TZM and MN's white region, while ANR maintained a high expression level, which may lead to the low content of cyanidin in them. Transcription factors MYBs, bHLH, and bZIP may play a key role in regulating anthocyanin-structural genes. The co-expression analysis showed that the meristem tissue may play a crucial role in the formation of the mixed white-red color in MN. Our study enriched the genetic basis of flower coloration differences in Yunnan Camellia which will be a valuable genomic resource to understanding the biology of coloration formation and for breeding the Camellia cultivars.

Integrative metabolome and transcriptome analyses reveal the coloration mechanism in Camellia oleifera petals with different color

BackgroundCamellia olelfera petals are colorful, and have high ornamental value. However, the color formation mechanism of C. olelfera petals with different color is still unclear. In our study, WGCNA method was applied to integrate metabolites and transcriptomes to investigate the coloration mechanism of four C. olelfera cultivars with different petal colors.ResultsHere, a total of 372 flavonoids were identified (including 27 anthocyanins), and 13 anthocyanins were significantly differentially accumulated in C. olelfera petals. Among them, cyanidin-3-O-(6''-O-p-Coumaroyl) glucoside was the main color constituent in pink petals, cyanidin-3-O-glucoside, cyanidin-3-O-galactoside, cyanidin-3-O-rutinoside, and cyanidin-3-O-(6''-O-malonyl) glucoside were the main contributors to candy pink petals, and peonidin-3-O-glucoside was the important color substance responsible for the red petals of C. oleifera. Furthermore, six structural genes (Co4CL1, CoF3H1, CoF3'H, CoANS, CoUGT75C1-4, and CoUGT75C1-5), three MYBs (CoMYB1, CoMYB4, and CoMYB44-3), three bHLHs (CobHLH30, CobHLH 77, and CobHLH 79–1), and two WRKYs (CoWRKY7 and CoWRKY22) could be identified candidate genes related to anthocyanins biosynthesis and accumulation, and lead to the pink and red phenotypes. The regulatory network of differentially accumulated anthocyanins and the anthocyanins related genes in C. olelfera petals were established.ConclusionsThese findings elucidate the molecular basis of the coloration mechanisms of pink and red color in C. olelfera petals, and provided valuable target genes for future improvement of petals color in C. olelfera.

Genome-wide detection of selection signatures in Jianli pigs reveals novel cis-regulatory haplotype in EDNRB associated with two-end black coat color

BackgroundJianli pig, a renowned indigenous breed in China, has the characteristics of a two-end black (TEB) coat color, excellent meat quality, strong adaptability and increased prolificacy. However, there is limited information available regarding the genetic diversity, population structure and genomic regions under selection of Jianli pig. On the other hand, the genetic mechanism of TEB coat color has remained largely unknown.ResultsIn this study, the whole genome resequencing of 30 Jianli pigs within a context of 153 individuals representing 13 diverse breeds was performed. The population structure analysis revealed that Jianli pigs have close genetic relationships with the Tongcheng pig breed, their geographical neighbors. Three methods (observed heterozygosity, expected heterozygosity, and runs of homozygosity) implied a relatively high level of genetic diversity and, a low inbreeding coefficient in Jianli compared with other pigs. We used Fst and XP-EHH to detect the selection signatures in Jianli pigs compared with Asian wild boar. A total of 451 candidate genes influencing meat quality (CREBBP, ADCY9, EEPD1 and HDAC9), reproduction (ESR1 and FANCA), and coat color (EDNRB, MITF and MC1R), were detected by gene annotation analysis. Finally, to fine-map the genomic region for the two-end black (TEB) coat color phenotype in Jianli pigs, we performed three signature selection methods between the TEB coat color and no-TEB coat color pig breeds. The current study, further confirmed that the EDNRB gene is a candidate gene for TEB color phenotype found in Chinese pigs, including Jinhua pigs, and the haplotype harboring 25 SNPs in the EDNRB gene may promote the formation of TEB coat color. Further ATAC-seq and luciferase reporter assays of these regions suggest that the 25-SNPs region was a strong candidate causative mutation that regulates the TEB coat color phenotype by altering enhancer function.ConclusionOur results advanced the understanding of the genetic mechanism behind artificial selection, and provided further resources for the protection and breeding improvement of Jianli pigs.

Density functional theory/time-dependent density functional theory investigations on the color-structure relationship of biopigment molecules

The study of molecular derivatives of biological dyes is of great importance for the green transformation of the printing, dyeing, and textile industries. In this study, B3LYP density functional methods are used to optimize the geometric configuration of the selected molecules and to explore the relationship between the structure and color of biological dye molecules and their color mechanism. This study focuses on the analysis of polyenes, quinones, indoles, and azide biological dyes in colors commonly used in the textile industry: blue, yellow, purple, red, green, and so on. Quantum chemical investigations show that the conjugate structure of a biological dye is directly related to its color and that the group or structure affecting the conjugate structure will in many cases cause a change in color. In addition, time-dependent density functional theory spectral calculations with the CAM-B3LYP functional for UV-Vis spectra show that in the visible band, the color of the remaining band after subtracting the absorption wavelengths was exactly the same as the color of the pigment. These results indicate that the color of the pigment is exactly complementary to the light absorption color of the material. Our study provides theoretical guidance for the design of molecular derivatives of biological dyes and is expected to promote the green transformation of the textile, printing, and dyeing industries to a certain extent.

Tracing the genealogy of research on the mechanism of blue flower coloration by anthocyanin based on Keita Shibata's work.

K. Shibata is the ancestor of the research on anthocyanins in Japan and proposed metal complex theory against the pH theory by R. Willstätter. Shibata's successors, S. Hattori and K. Hayashi, made efforts to clarify blue flower coloration by metal complexation and found commelinin, a self-assembled supramolecular metal complex pigment, in blue dayflower, Tsuyukusa. The author introduces two key reports on blue flower coloration published in the Proceedings of the Japan Academy and describes the subsequent development of the study.

Anthocyanin gene enrichment in the distal region of cotton chromosome A07: mechanisms of reproductive organ coloration.

The biosynthesis of secondary metabolites like anthocyanins is often governed by metabolic gene clusters (MGCs) in the plant ancestral genome. However, the existence of gene clusters specifically regulating anthocyanin accumulation in certain organs is not well understood. In this study, we identify MGCs linked to the coloration of cotton reproductive organs, such as petals, spots, and fibers. Through genetic analysis and map-based cloning, we pinpointed key genes on chromosome A07, such as PCC/GhTT19, which is involved in anthocyanin transport, and GbBM and GhTT2-3A, which are associated with the regulation of anthocyanin and proanthocyanidin biosynthesis. Our results demonstrate the coordinated control of anthocyanin and proanthocyanidin pathways, highlighting the evolutionary significance of MGCs in plant adaptation. The conservation of these clusters in cotton chromosome A07 across species underscores their importance in reproductive development and color variation. Our study sheds light on the complex biosynthesis and transport mechanisms for plant pigments, emphasizing the role of transcription factors and transport proteins in pigment accumulation. This research offers insights into the genetic basis of color variation in cotton reproductive organs and the potential of MGCs to enhance our comprehension of plant secondary metabolism.

Combined transcriptome and metabolome analysis revealed the molecular mechanisms of fruit skin coloration in pink strawberry.

Elucidating the key genes and metabolites responsible for fruit skin color is essential for the breeding of strawberry varieties with beautiful fruit color. Here, transcriptome and metabolome analyses were used to identify the key genes and metabolites associated with fruit skin color in strawberry accessions of red skin (Kaorino), white skin (2012-W02), and the pink skin (Fenyu NO.1, the F1 hybrid of Kaorino and 2012-W02). The metabolomic data showed that the content of anthocyanin-related metabolites, such as p-Coumaroyl quinic acid, 5-Hydroxyconiferyl alcohol and Coumestrol were significantly higher in red-skinned strawberry line Kaorino than in the white-skinned line 2012-W02. The flavonoids and isoflavonoids such as syringetin and 2,7,4'-trihydroxy-isoflavone, were less expressed in the Kaorino than in the other two accessions. Transcriptome analysis revealed that the expression of genes involved in anthocyanin biosynthesis, such as BZ1, F3H, CHS, CHI, DFR, 4CL, PAL, CCR, 4CL, F5H, REF1 and UGT72E, were also significantly upregulated in the red-skinned line Kaorino compared to the white-skinned line 2012-W02, while the HCT, CYP75B1, FG3, HIDH, IF7MAT, I2'H, and VR was downregulated in Kaorino. Combined transcriptome and metabolome analyses revealed that the pathways of isoflavonoid biosynthesis and flavone and flavonol biosynthesis, and the phenylpropanoid biosynthesis pathway essential for anthocyanin synthesis were commonly enriched by DRMs and DEGs. In addition, the metabolites of peonidin 3-O-glucoside, 2'-hydroxydaidzein and daidzin, and the genes of CYP93B2_16 and UGT73C6 were detected and most accumulated in pink-skinned Fenyu NO.1. This result suggested that the main strategy for obtaining a red skin color is to enhance the upstream pathway of anthocyanin biosynthesis, including the phenylpropanoid biosynthesis pathway, and to restrict the downstream steps in the flavonoid biosynthesis pathway, such as the branch pathway of flavone and flavonol biosynthesis and isoflavonoid biosynthesis.

Genetic analysis and preliminary mapping by BSA-seq of the CmSR gene regulating the spotted rind trait in melon (Cucumis melo L.).

Melon (Cucumis melo L.) is an economically important horticultural crop. Spotted rind at maturity is an important appearance quality trait in melons. However, the gene controlling this trait remains unknown. In this study, the inheritance pattern of this trait was explored, and the candidate gene underlying this trait was also successfully identified. Genetic analysis showed that a single dominant gene, Cucumis melo Spotted Rind (CmSR), regulates the spotted rind trait. A preliminary genetic mapping analysis was conducted based on a BSA-seq approach. The CmAPRR2 gene was identified to be linked with the spotted rind trait and was located on the short arm of chromosome 4. It harbored two single-nucleotide mutations (chr4: 687014 G/A and chr4: 687244 C/A) in the non-spotted line 'Yellow 2', which may result in the alternative splicing of the transcript and an amino acid change in the respective protein, from proline to glutamine, respectively. Moreover, marker SNP687014-G/A was developed and co-segregated with the spotted rind trait. Therefore, it is speculated that the CmAPRR2 gene may be involved in the regulation of the spotted rind trait in melon. This study provides a theoretical foundation for further research on the gene regulatory mechanism of the rind color in melon.

Adaptive coloration enabled by the reversible osmotic annealing of chromatophore-like microcapsules

Generating dynamic color change in an adaptive, bio-inspired microcapsule that can be activated to reversibly switch between pigment-based and structure-based coloration mechanisms.

Deciphering colour mechanisms in co-crystals and salts containing violuric acid and chosen l-amino acids

Salts and co-crystals of violuric acid with l-amino acids (l-His, l-Lys, l-Trp, l-Tyr, l-Ser) as novel chromic materials with potential as sustainable dyes.