I do agree good food is a must. I started giving my fish a wider variety of foods (zooplankton, formula 1 pellets, etc...) and the fish look better. I had a maroon clown with brooklynella (clownfish disease) and couldn't catch the dang thing...so I gave up, and just started feeding him better foods, and bam he cured right up, after suffering for months. The formula 1 sure seemed to help him but I agree, do a search on best foods for angelfish/flame angels, that should give you some ideas.
I have a flame angel myself, but it's pretty new...so I'm still experimenting w/different foods. I may look into the angelfish food containing sponges...sounds like it would brighten their color. Good luck!