It's just the nature of the beasts. Black goldfish are particularly likely to change, but changes can occur in any color. Environmental factors, like nutrition, can affect the intensity and quality of color, but don't appear to have much to do with changing from one color to another.

I brought one of my black color goldfish in for the winter since I just set up a new aquarium. Went from black to golden in a day or two.

Hi
I got back into goldfish in Feb. due to a freeze of my tropical fish. Fearing mosquitoes after ths total loss I put some "feeder" goldfish in the 5x10 pool. Anyway they spawned ,I'm guessing 3 times judging fron the sizes of the offspring. When about an inch long had nmany different colors including brown and black. The older ones are now either completely gold or white and gold. Some complete whites . Have also noticed some are growing the long flowing fins not displayed in the originals. these are also becoming very white in the fins only. a couple have fins twice body length already.
Amazed at the variety ,considering the parents are just very ordinary solid colored short finned .typical goldsish lol Don't know what I'll do when they spawn again as I'm guessing there are well over 100 in less than a year.
I was never able to keep GF in the past due to Herons
But if they're catching some it's sure not obvious!!! lol. gary

Water quality and temperature also has an effect on black goldies.