I had a fish like that, she was a quite small full grown fish that I had for a reasonable amount of time, a pretty yellow and black spotted fish. She was in the public tank so I'm not sure if it was because she was pregnant, but she never got fat or anything and my smaller strain of guppies tended not to get pregnant, so that was highly unlikely. For a while before turning white, she wouldn't eat, and just swam around at the bottom of the tank, not doing anythink in particular. She seemed healthy enough, except for the eating part. Then my family had to go to the US for a home visit, (he was an expat in China) and I couldn't do anything about her eating, because even though we had somebody come over and take care of the fish, I knew she wouldn't eat either. I was sure she'd be dead when we got back. Miraculously, when we got back she was still there and very much alive, but I got a huge shock: she was all white on her body and her beautiful yellow and black spotted tail had turned into a light red-ish color, matching her fins and her eyes were red too. I had no idea what had happened, but there was no change in her appetite, still not eating. I though of the rabits at the pet store, who were pure white with red eyes, and I thought she had turned albino. She died after we put a couple of mini orange and blue crayfish in, (we were assured they wouldn't harm the fish) and their first meal was her. To this day I'm still looking for the cause of guppy color change.