Yes, their color is natural, but I was saying that people "enhance" their natural coloration by supplementing the diet of a variety of organisms with carotenoids, etc. It "brightens" their natural colors. Goldfish, like all fish, have cells that are genetically programmed to reflect a certain range of colors at maturity (chromaphores and melanophores), but diet has a lot to do with whether or not they'll be able to (and how intensely they will) express that coloration.
Carotenoids are made by plants (aquatic and terrestrial). The carotenoids are eaten by zooplankton where they become concentrated (they're lipophyllic). They accumulate at each higher level in the food web, and not coincidentally, they're added to almost every flaked fish food. If you see "xanth..." as an ingredient, it's providing the material to enhance yellows and browns; "cyan..." enhances blues & greens; shrimp or "carotenes" enhance red, orange, yellow; "porphyrins" normally enhance green, but can become red (depending on their oxidation state); "flavinoids" provide blue, red, and purple and so on.
I've had direct experience with this phenomenon during research toward my PhD in Biological Sciences. I've attached two pictures of fish I've raised that show how profoundly diet can influence coloration. Remember, most people would consider both of these fish to be "naturally" red & blue during reproduction, but diet and genetics both play major roles. Look at the photo gallery for more details on their dietary differences. This is the same phenomenon that causes flamingos to be whitish in captivity (unless they're supplemented with dietary carotenoids) and pink in the wild.
Rick
