Black goldfish changing colour ; is it normal?

I have bought a black goldfish 7 months ago . It has no health problem and the temperature and the atmosphere are normal . But, my black fish has turned gold recently. I have never seen such a thing before...Can anyone tell me if it's a disease or something or it's normal??

 

Many goldfish will do this as they are not "pure" bred, and might have traces of say calico, red or black moor in them... Eventually other colors can come out...

 

Goldfish have an unstable pigmentation and can change color (when they are older)

I read in a forum that someone did experiments with lighting bulbs in the tanks that had goldfish with faded or changed color
He used some bulbs with a lot of red spectrum in them and his goldfish started getting back their color slowly and eventually it came out better than it was before! He posted pictures of their progress too..
I am trying to find the link at the moment..I will report back if i find it

 

He is fine. Goldfish change colors a lot.. especially black ones.