Some fish can change colour as they grow. This is normally caused by genetics and having different coloured parents or ancestors. Black moor goldfish are also available in red, white, calico and panda colour forms and if one of the parents had any other colour besides black, it can show through as the fish grows.
Same deal with the molly except they normally come in yellow, silver, green, black, dalmation (black & white spots), and a few other colours. If one of the parents was not pure black, then the offspring can show some colour.
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If a fish develops cream, white or grey patches or film over all or part of their body, (this can make them look cream, grey or white), then this is a health issue and needs to be dealt with asap or the fish could die.
Cream, white or grey patches over part of the head, body or fins are caused by external protozoan parasites. Salt will usually treat this.
A cream, white or grey film over the entire head, body and fins is caused by poor water quality and a dirty environment. A big (75%) water change and gravel cleaning the substrate every day for a week will usually fix this problem.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
