Gold Fish gone Silver

[Gollom122]

I changed to a larger tank about 6 months ago after fully cycling the water. My 2 goldfish (comets I believe) were transferred and have been much more active in their new surroundings. However over the past 6 weeks one of them has lost all his Gold (He used to have a silver underbelly) but has now lost almost all of it except a very small bit is left. Attached photo before and after.
Is this anything to be concerned about?

 

[jlk]

Normal. Not all goldfish colors are static. As long as they healthy, happy and they have a sufficient sized tank, I would not worry.

 

[Ursi]

Wow! That's amazing!

 

[Wiggles24]

Goldfish can change colors it's normal