Some goldfish varieties have been known to change colors. For example a blackmoor that is supposed to be black can turn to orange over time.  Although this is not normal per se , it shouldn't harm the goldfish either.

Some of the reasons that could cause the color of the goldfish to become dull or fade could be a fast drop in pH level in  short amount of time, parasitic infestations, and recent use of antibiotics due to an illness. Goldfish colors can also fade if they are kept in the dark for a while (but this is only temporary) and diet can also play a role.