Abstract
BackgroundA variety of techniques are used to study the colours of animal signals, including the use of visual matching to colour charts. This paper aims to highlight why they are generally an unsatisfactory tool for the measurement and classification of animal colours and why colour codes based on HTML (really RGB) standards, as advocated in a recent paper, are particularly inappropriate. There are many theoretical arguments against the use of colour charts, not least that human colour vision differs markedly from that of most other animals. However, the focus of this paper is the concern that, even when applied to humans, there is no simple 1:1 mapping from an RGB colour space to the perceived colours in a chart (the results are both printer- and illumination-dependent). We support our criticisms with data from colour matching experiments with humans, involving self-made, printed colour charts.ResultsColour matching experiments with printed charts involving 11 subjects showed that the choices made by individuals were significantly different between charts that had exactly the same RGB values, but were produced from different printers. Furthermore, individual matches tended to vary under different lighting conditions. Spectrophotometry of the colour charts showed that the reflectance spectra of the charts varied greatly between printers and that equal steps in RGB space were often far from equal in terms of reflectance on the printed charts.ConclusionIn addition to outlining theoretical criticisms of the use of colour charts, our empirical results show that: individuals vary in their perception of colours, that different printers produce strikingly different results when reproducing what should be the same chart, and that the characteristics of the light irradiating the surface do affect colour perception. Therefore, we urge great caution in the use of colour charts to study animal colour signals. They should be used only as a last resort and in full knowledge of their limitations, with specially produced charts made to high industry standards.
Highlights
A variety of techniques are used to study the colours of animal signals, including the use of visual matching to colour charts
Perhaps most worryingly, whilst some printers showed a fairly constant increase in reflectance as the R or G/B value increased, many printers produced charts where there were sudden large jumps between what should have been equal steps between the colour blocks, or had several colour blocks at the top end of the RGB values having very similar reflectance (Fig. 2), showing that some printers are constrained to smaller variations in colour spacing
The experiments detailed in this study show three important results with respect to the use of printed colour charts to identify the colours of animal signals
Summary
A variety of techniques are used to study the colours of animal signals, including the use of visual matching to colour charts. Birds typically have four single cone types, compared to three in humans, and unlike humans, most birds are capable of perceiving light into the ultraviolet spectrum [[17,18,19,20,21,22,23,24], reviewed by [25,26,27,28]] This means that birds should be capable of perceiving a wider range of hues, and will differ from humans in the magnitude of perceived colour differences, even for those spectra visible to humans
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