bilin, any biological pigment (biochrome) belonging to a series of yellow, green, red, or brown nonmetallic compounds that are formed as a metabolic product of certain porphyrins. In addition to their presence in the bile pigments of mammals, bilins also occur in invertebrates, lower vertebrates, red algae, and green plants. Bilin pigments not only impart various colours to certain animal parts or products but are also indispensable in a number of photoperiodic processes in green plants and as accessory pigments in the photosynthetic process in red algae. As accessory pigments, bilins absorb photons of wavelengths not absorbed by chlorophyll pigments.