Note that this is only my opinion, and should not be treated as a definite answer.
There is a question that always seems to be asked. it is "How do you get your goldfish's colors so bright?" Even I have asked this question on many occasions. the answer may be simpler than you think. First off, lets start out with the colors of goldfish. the colors are created by the cells called chromataphores that contain pigments. there are 3 types of pigments. These pigments are xanthin (yellow), erythrin (red), and melanin (black). The combination of these pigments are what cause the different colors in fish. Just a few examples, the mixture of the red and yellow pigments cause the ever so common orange. the mix of black and yellow create a brown color. A fish that is all white will simply have no chromataphores. Depending how deep these pigments are in the fish's scale and skin tissues can change the appearance of the colors and also the stability. Black pigments that are deep in the tissues will often give off a blue appearance. Pigments that are deeper in the scales and skin are also a lot more stable compared to the ones that are near the surface. Black colors that are near the surface seem to be the most common. This is why most fish loose their black coloration. it is simply unstable.
Goldfish also get a lot of their coloration from their environment, unlike other fish that are born with pigment. What the fish eats can strongly influence to colors of the fish. A fish that would be fed low quality cheap foods all the time would have a very pale appearance. This is because the cheap foods don't contain very many pigments that are in the form that fish can absorb to maintain the pigments it already has. There are a variety of good high quality foods on the market that contain pigments that the fish can use to maintain it's colors. This is why fish that come out of green water are always have nice bright colors. The green algae contains pigments that the fish can absorb to get highly desired bright colors. I often feed my fish shrimp and blood worms because these foods are high in usable pigment. Another is the general color of the environment that the fish is living in. If the colors are dull, the fish will often try to make itself as dull as possible. Its kind of like a form of camouflage. if you were to put the fish in a bright or darker environment, the fish will make its colors bright as possible. this is why fish are displayed in blue tubs at shows. Lighting can also affect the brightness of the environment. This is why certain fluorescent bulbs are used in tanks. An example of lighting that brightens the environment, in this case a tank, would be the Aqua-Glo bulbs from Hagan. these brighten up the environment making your fish try to blend in by brightening itself. Finally, the last thing that can influence the colors of a fish is water quality. Red pigments are usually more visible in softer more acidic water(low ph), while black pigment is more visible in harder and more alkaline water(high ph). Poor water can also cause a fish to fade. This is because the pigment in the chromataphores groups together, as in good water it spread apart. Ive experience this myself. Since i am starting a green water setup, the ammonia levels are a bit high, and the fish is a lot paler than it was yesterday.