It is interesting, some Koi do change their colouration as they mature and dependant on the amount of sunlight they are exposed to. I suppose you can say it's equilivant to us getting sunburn. I have looked this up and all my books and resources tell me if your fish does not seem to be ailing at all, then leave alone. Although something like stress-coat maybe added to the pond to give the skin a slime coating. Fish naturally exude slime as a barrier against infection, if the slime has been dried up due to too much sun exposure,they get burned just like us, so replacing the slime in the form of stress coat may prevent any further changes to this or the other fishes you have. Stress isn't just crowding or infighting, there are many causes for stress, and sun exposure is just another one.
