Colour Loss in Comet

Hi,
Just wondering what could cause a major colour loss in a goldfish (comet) 2
nights ago one of my comets which I have had for about 4 years last lost
nearly all its colour. It is a pretty big comet nearly 20cm (8inches) in
length.

I have checked my water as I normally do and everything is just fine, the
fish is still active, swimming normally the other fish is leaving it alone,
its still eating normally.

Should I be worried?

cheers
Scott