Coloration change has nothing to do with diet but rather the fishes mood, oscars will run like a kaleidoscope based on their current temperment. Plus what a Oscar looks like now will have no resemblance to their final coloration as adults. This color morph can go from pale to black in seconds depending on his/her current feelings, this is one of the reasons they're referred to as wet pets since they actually show feelings and personality. Feed him/her a quality diet and keep the water parameters clean with a nitrate concentration below 20ppm and you'll have that fish for 10+ years, unforturnely a good portion of oscars sold today did from fatty liver disease and head and lateral line errosion(HLLE).