Flakes have nothing to do with it. It's more or less the water quality. Mine are fed a diet of flakes in addition to some frozen brine shrimp and blood worms. They have great colors. They're also in a tank with white sand.
They don't seem to color as nicely when younger.
My advice would be to buy them, quarantine them for a month, and then introduce them to the main tank. I've had mine for 2 years now. They're almost 5" long and gorgeous.