Diesel fuel, which is widely used in engines, generators, and industrial equipment, is often adulterated with kerosene. The use of adulterated diesel fuel reduces the performance and durability of diesel–fuel-powered engines, causes environmental pollution, and promotes tax evasion. However, detecting kerosene in diesel using a high-precision instrumental technique such as gas chromatography with flame ionization detection is difficult because both diesel and kerosene contain C6–C16 paraffins. In this study, we developed a simple and reliable colorimetric sensor to detect various volumetric fractions of kerosene in diesel. A novel 15-layered one-dimensional photonic crystal (1D PC) was prepared using a photo-crosslinked butyl acrylate (BA)–based copolymer [P(BA-co-BPA)] as the low-refractive-index material and a photo-crosslinked copolymer–TiO2 hybrid (Ti70) as the high-refractive-index material. The P(BA-co-BPA) layers swelled to a greater extent in smaller-sized C6–C16n-alkanes than in the larger ones. Consequently, the reflection colors of P(BA-co-BPA)/Ti70 PC underwent a greater red shift from the sky-blue color of the pristine state as the swelling increased from the C16 to the C6n-alkanes. When immersed in pure diesel, the 1D PC exhibited a green color, which underwent a greater red shift as the volume fraction of kerosene increased from 10% (∼limit of colorimetric detection) to 100% in the kerosene + diesel mixtures. The 1D PC reached saturation within 45 min and changed color from the sky-blue corresponding to the pristine state to greenish yellow, yellow, yellow-orange, orange, and reddish orange in 10%, 20%, 30%, 50–70%, and 100% kerosene, respectively. Moreover, it exhibited recyclability in five swelling/deswelling cycles in diesel and kerosene. We expect that these findings will promote the development of simple and state-of-the-art 1D PC sensors.