One of the most misunderstood things in the field of nutrition are the carotenoid pigments as they are the least understood in the field of nutrition.

Colors in fish is determine by the Chromatophores involved. Chromatophores are specialized cells which can contain or produce pigment, or reflect light in a specific way to create a certain desired hue.  

Pigments are characterized by their colors.
Carotenoid pigments are red and orange.
Xanthophylls are yellow.
Melanin pigments are black and brown.
Phycocyanin is the blue pigment derived from blue-green algae.
Cells containing yellow pigments overlying those containing blue pigments can produce green hues.

Fish are capable of producing some pigments, but others must be supplied in the diet. Black and brown pigments are produced in cells called melanocytes. Fish are incapable of producing carotenoid and xanthophyll pigments on their own. Therefore, these must be supplied in the diet this is the reason why those colors, even if they are not showing to begin with can be brought out through feeding.

I have recently developed a frozen beef heart formulation which will restore the colors of faded fish. Some examples are Red Discus, Red Czech Angelfish and Koi Angels.  This food will only bring out the red, orange and yellow colors in fish that are genetically pre disposed to that color spectrum as it will do nothing for the color of a black angelfish. However, the same additives in this food are considered to not only be a color enhancer but a growth promoter, immuno-stimulant and to increase fertility.