In the UK we have a lot of rainbow trout fisheries which stock a small number of blue and also golden trout. These are rainbow trout that have been selectively bred to loose one or more pigments so they are no longer rainbow just blue or gold.
EDIT: Link added
I assume this colour variation through lose one or more pigments can also occur naturally and in most species.
Went fishing yesterday, 44 degrees, lots of bites but only one small snapper landed.
Duane B