Carotenoid-based colour signals are widespread in the animal kingdom and common textbook examples of sexually selected traits. Carotenoid pigments must be obtained through the diet as all animals lack the enzymatic machinery necessary to synthesize them from scratch. Once ingested, carotenoids are metabolized, stored, transported and deposited, and some or all of these processes may be limiting for signal production and thus subjected to social or sexual selection on phenotypic coloration. Very little is known about which genes and physiological pathways are involved in carotenoid pigmentation which is unfortunate, as genetic information would allow us to investigate the biochemical consequences of sexual selection. In this study, we present a transcriptome-screening technique and apply it to a carotenoid-signalling bird species, the southern red bishop Euplectes orix, to uncover the gene(s) responsible for the conversion of dietary β-carotene (orange) to canthaxanthin (bright red). The transcriptome, extracted from the liver of a male entering his breeding moult, is expressed within bacterial cells genetically modified to synthesize beta-carotene. Effects of expressed E. orix proteins on the structure or amount of β-carotene are initially detected by eye (based on colour change) and subsequently confirmed by high-performance liquid chromatography. Here, we demonstrate the validity of the technique and provide a list of candidate genes involved in the carotenoid pigmentation pathway. We believe that this method could be applied to other species and tissues and that this may help researchers uncover the genetic basis of carotenoid coloration in vertebrates.