goldfish is very easy to change color, normally black to red, or yellow then white. So sometime you can see very beatuigul interesting bicolour fish, some of them can not stay that way, will change and fade in a few months.
But some special kinda cultivated fish will keep the colour, e.g like panda, which is black and white, the black should appear on the fish body at the right pattern and should not be fade. However, most panda, esp poor quality one will turn to white in 2 or 3 years, if you want to avoid it, try to keep it in green water, with a lot of sun light, it helps.