People often ask why their goldfish has changed colour and there can be several explanations. The most popular colour change is from black to red. Sometimes people buy a fish because it has nice black marks on its otherwise normal colour of orange. The black disappears and the fish ends up pure orange. This is a normal colour change, as all goldfish, start life black and turn to orange as they grow.
Age and temperature affect the change as can food. Some foods, especially koi foods have a colour enhancer that is based on carotene and brings out the reds and oranges in the fish. Sometimes goldfish turn white and this can be compared to hair turning white as people get older. Some people’s hair turns white quite young and others never do. This is the same with goldfish with most stay orange all their lives.
Black Moors sometimes turn orange like a goldfish. This is because the fish is a genetic throwback and has not bred true to the Black Moor colouration. Very occasionally a goldfish will develop small black patches as it gets older. This not natural and could well be due to a tiny parasite which will slowly spread if left untreated and may eventually kill the fish. An ordinary parasite treatment should cure the problem.
Other fish will change colour as a normal development when the fish matures. Baby Shubunkins start life as a very pale fish with no colour and gradually assume their characteristic red blue and black colours. Koi carp also develop better colour markings as they get older. This is why it is always a bit of a gamble buying small Koi as you never know what colour they will finish up when they have matured. Indeed Koi enthusiasts are always searching for the perfectly marked Koi for their collection. They sometimes go to extreme lengths to achieve perfection, even to the point of surgically removing scales to remove small blemishes in the markings.