Mineral content of the water can really do weird things to fish. I don't see this as a mineral thing, but as has been suggested its more than likely a genetic or dominant gene thing. Different though.
Lines, I had an old sand pit that had crappies of unusual size in it and fished it a lot when I was much younger. I had to park on a county road and walk in to fish by the owner's rules. One winter the property changed hands and I got the tin star escort out of there and never got back in again. The black crappies in that green watered pit had an eerie blue color over their black markings. The water had a heavy blue clay influence that made the crystal clear water appear a green-blue. Crazy colorations on the crappies.
The Mississippi River's back waters in a couple of pools I fish have black crappies that are darned near yellow on the white markings. A half mile away on the open channel and black crappies look just like black crappies should. I guess that creatures inherit their environment more than we think.