colour enhancement.

All the carotenoids have effects on red to yellow colors. Do a www.google.com search for "astaxanthin " It's what makes cooked lobster shells pink. It's in krill. Color-enhancing flakes contain astaxanthin, probably from ground krill. There's good info on astaxanthin here: http://www.astaxanthin.org/astax.htm cool to know!

The only colors that can really be enhanced are reds, oranges and yellows. Blues and greens are "structural" colors that result from the diffraction of light ("Tindall effect") among layers of guanin crystals. (Alone, they're responsible for silvery and white colors.). Black is from melanin, which affects all the other colors, but its production isn't really affected by diet.

Methyl testosterone can "juice up" young fish, to give them sexually mature coloring, and make female gouramis as colorful as males-- and sterile. You're not talking about hormones, though, I don't think.

A little astaxanthin is harmless fun.

Well, my first thoughts go to the types of fish you have. In my experience, feeding your fish a rich and varied diet goes a long way to improving their color, bringing out the lovely bright colors we like to see.... Also, the gravel, plants, backgrounds, etc. can play a pretty big role in bringing out a fish's color; for example, a neon that looks washed out and pale in a stark-white and green tank really GLOWS against a black background. I've also seen food designed to enhance colors, and I have used it, but only seen limited success. The change of environment seems to work best for me....