The liver is darker than the kidneys possibly because of the higher content in deoxygenated blood, bile pigment, and lipofuscin (discussed in the "Lipochromes" section) (Figure 2, D and H).
(47) We now appreciate that colors in living organisms are the result of complex biochemical reactions with the production of biologic pigments (cytochromes, porphyrins, melanins, lipochromes) or because of structural coloration (iridescence).
prosthetic group) Iron Brown-black, bound to hemosiderin or ferritin, ferric state (Fe+3) Lipofuscin and Brown-black other lipochromes Autofluorescence Known as "wear and tear" pigment Melanins Eumelanin Brown-black Pheomelanin Golden yellow-red Neuromelanin Brown-black Intrinsic Unknown neuronal pigment?
They found that all the birds' red feathers contained a set of five pigments called polyenal lipochromes.
Unlike carotenoids, which birds obtain from their diets, lipochromes appear to be synthesized within the follicles of maturing parrot feathers, says McGraw.