My Blackmore is changing colours! Help?

I have a fishtank at home containing 2 goldfish and 2 blackmores, 1 of my blackmores over a period of time has started to change colour and is now completly orange except form the rim of his tail, around his eyes and under his mouth there is a line that almost looks like a moustach. Have i done something wrong or is this normal, if so what is happening?