I think what you're looking for is really any high quality food that contains astaxanthin. For the best colors and healthiest fish you're going to want to supplement their diet with live/ frozen foods as well.
Carotenoids add color to your fish, as @Plinkploop mentioned astaxanthin is one very common carotenoid, spirulina contains some very good stuff, and another is carotene. There are actually quite a few. Here is a whole paper on it.
Anyway your best bet is going to your local fish store and reading labels on the 'color enhancing' fish foods looking for carotenoids in the ingredients and hopefully finding more then one or two. I like bug bites (which only list their ingredients online somewhat annoyingly), and omega 1. There are MANY others. If you want to dive into the live food world there is a lot you can do to bring out color, but you need to devote space/tanks to raising live foods. This is definitely easiest outside but can be done indoors as well.