Some of the best foods to use that will bring out the color in those types of fish are food like:
-Omega One Super Color flakes, or pellets (depending on what the fish are used to)
-Almost any Frozen foods (brine shrimp, blood-worms, beef-heart, etc) Since frozen food is really close to their natural diet I have found that it really brings out the color of my Cichlids, Tropical fish, and so on. Also the higher the protein content and lower the moisture level the better the frozen food is, but if you don't like to mess with Frozen food then i would highly recommend the Omega one brand of food. Really if you find a food with a high protein level and the first ingredients start with things such as Whole herring, Whole cod, etc. then it should really bring that color out.