Fishes get their colors from pigment cells. Xanthophyll pigments are yellow, Melanin pigments are black and/or brown, Carotenoids are orange and/or red, Phycocyanin is blue. Fishes cannot produce some pigments such as carotenoids and xanthophylls, therefore it must come from their diet.
The common practice of fish food manufacturer is to lace the foods with artificial colorant or dye especially of the red kind. Once consumed by the fish, it will take the place of carotenoid pigments in the cells beneath the scales, thus displaying the red color. This artificial dye will wreck havoc to the endocrine and nervous system, both that influence coloration in fish. Because of this, the production of certain pigments such as melanin goes hay-wire and black and/or brown pigments will show up at the tail or some parts of the fishes’ body, making the fish look “dirty”.