I feed Omega One to my Angels. It's most important to give them a quality food. NLS is a very good food, and I would just stick with that.
Reducing stress quite honestly will give you the best color. I have a couple of the 'zebra' variant, and when stressed or in bright lighting their black bars go invisible and they look more silver. When they're nice and happy, and not under bright lights there bars darken up amazingly. It literally is like night and day!
A natural habitat for these guys (and gals) would be lots of branches and floating plants, with a sand substrate. The floating plants should be thick, so the light below is minimal. They really prefer a dim environment. I don't keep mine too dim, as I have regular substrate plants that need some light, but I do still keep a floating cover for a 'roof'.