My comet was a beautiful bright red/orange, my mum would say how beautiful and big and healthy he looked. Not so long after(when he was about three) He changed to a white colour! It was almost as if it happened over night!
He still looks SO healthy though. I thought I had done something wrong too, but I looked it up on the internet, and although they can turn white because of lack of light, some would do this no matter what the conditions they are kept in.
I have had other goldfish turn white too!