It doesn't look like a disease problem to me. It looks more like nerve damage that won't permit the chromatophores from expanding/changing colors. Other than not looking perfect, it shouldn't cause your fish undue problems.
Mike - did you miss post #4 in which he said the fish has lost its left fin? If so i think this is more than just a chromatophore nerve control issue. But if he's still eating well, there's hope.
The fin stump is close and no more fin rays.If it was bitten off I'd say yes it could grow back, but if an infection or tumor destroyed it, then I doubt it. How does the fin stump look? Open sore or healed over? Are there still stubs of fin rays, or is lost all the way down to the fleshy base of the fin?