As for color enhancing foods im not sure. I have heard some types of fish eggs bring out colors in fish, Lobster eggs i think. Anyway, how big are these juvies? I fed my betta frozen blood worms, and frozen brine shrimp. He was an adult betta. If they are closer to betta fry newly hatched brine shrimp i beleive is whats best. But really thats for very small fish. As for store bought foods go, read the label and see what they are using to enhance color i would stay away from any artificial junk. Most betta flakes or pellets should be a good feed, as long as they take to it. If they dont, go get some frozen stuff. i doubt they'd turn that down.

good luck