Color loss is an attribute of stress, which may be the case of the goldfish is not housed in a large enough tank (ie pretty much anything under 80L).
The loss of pigment is also a perfectly natural process in goldies, ie jet-black goldfish turning orange or orange and white goldfish turning white. Here is a good article on the process of 'cellular suicide' in which the pigment cells in goldfish die with age, making the fish lighter.