Hi,
From my experience importing fish there are several factors that affect a fishes colouration.
1. Sex
2. Dominance
3. Health
4. Size
5. Geographic region
6. Food
When it comes to food the best source for aquarium fish is a frozen marine one. When I wholesaled fish I used to feed the Pacifica Plankton. I found that the vast majority of fishes readily took to it. Even just after being unpacked from the bag. I jokingly call this food "Fish and Coral crack" because they do get addicted to it. When I feed the fishes in my coral tanks the corals all put out their feeding tentacles... like they are begging for it. For more info on the food you can check it on my website at:
http://www.oceanfreshaquarium.com/pacifica-plankton/
Cheers,
Tim