Best possible color is a simple formula: clean water, including water changes, filter maintenance, and regular cleaning of gravel if you have gravel, and a premium food with few grain ingredients and zero or very few ingredients like: corn, brewers yeast, gluten meal, rice, potato, feather meal, soybean meal, "middlings", gluten products, "starch" as a listed ingredient, etc.
You may not be overfeeding the tank overall but that fish is absolutely overeating. You never want to see a belly like that, it means trouble. The only bulging belly you should see, including immediately after eating, would be a female ready to spawn. An occasional small, temporary bulge after eating is not ideal but acceptable, more so in fry or small fish, but an extreme bulging belly like that is just bad news-- unless it's a female very full of eggs and a spawn is imminent.
It happens sometimes, an individual fish in a tank is a pig. But if he keeps it up he's likely to bloat-- if he hasn't already started. Best thing you can do for him, if possible, is isolate him and fast him for a while, minimum a few days, then feed only lightly until that belly is gone. Sometimes doing that cures a greedy eater and they can go back in the main tank, with some of them you have to do it more than once, occasionally one needs to go where you can feed the whole tank lightly on a permanent basis-- All of this is to save the fish from eventual illness or bloat.
Healthy fish in good water will grow just fine with light to moderate feeding of a good quality food.