I agree Mike that K.L. is seeing something other than color. But he could be seeing a shimmie. If it is a shimme, then I suspect it is of environmental origin and not of genetic expression ( that is actually impossible). There are environmental reaction skimme that sit on a koi's epidermis much like dots of black tree sap sit on the surface paint of a car.
I don't think anyone knows if these type simme are a cllection of minerals or the like ON the skin or a byproduct of the metabolism ( waste, hormones) that pushes material up and out of the system as the the skin sheds ( yes, koi shed slime and epitheal cells). Either way, what I do know is that they are very easy scraped off -sometimes just with the flick of a finger nail. JR