I use TetraMin flakes (some old stuff I have), O.S.I. Spiurlina Flakes, and Freeze-Dreid Blood Worms. I haven't had much problem with color and the fish seem happy
I use Omega One color flakes along with other varieties and my fish look great. Many of the foods add krill to help bring out the coloring in fish. Omega One uses salmon skins. As long as you use a good fish food and give them variety with other foods, your fish should be healthy and beautiful. Look for fish food that doesn't have fishmeal as the first or second ingredient on the can. Those are your better quality foods.
I use O.S.I. It appears that for some fish (like my late tiger barbs) This is great for their coloration other fish do better with other foods. The best way to "enhance" colors is frequent water changes.
Yes, flakes or foods that contain astaxanthin act in a way similar to beta-carotine: they enhance the colors in the red spectrum. Reds, yellows, oranges, even browns.
Yes, it works I feed special foods that contain NatuRose, which is a source of astaxanthin, to all my fish and my Boesemani rainbows are VERY colorful.
I feed a goldfish and a butterfly koi with ColorFin granules, and it was a matter of 2 weeks until the Gf became bright red and the koi went from pale to golden.