Color also have to do with genetics, if the colors arent in their genes, no matter what you feed it, that color won't show. Also if you want colors of your fish to pop, you'd have to have contrast with the backround too. To answer your question, something with krill or astaxanthin will make reds and other colors pop because it is a carotenoid. Fish food with high amounts of carotenoids can help enhance color. Also spirulina can help a fish's color, even though its not a carotenoid . But one of the biggest factor is the health of your fish, the healthier the fish, usually the brighter the fish.
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