I just want to point out that Koi do not lose their color with size or age typically.
they can if they have bad genetics, poor quality food and with different water temps
It isn't losing their color though, some fading yes but not completely losing color.
Most color in Koi is dictated by genetics, then sun exposure and food etc. Yes color may change over time but they do not lose it. Some fish lose specific colors going from orange and white to white. Or gaining some black, usually spotty, or fade in brightness. Even a faded orange koi is still orange however. In fact faded fish can often times be perked up and the color brought back into vibrancy.
They DO NOT go from say white or orange to Grey and simply lose their color to the point in which they would simply look a green/grey or revert back to common carpness. Unless of course the fish was a green/grey koi to start with heh..then it is quite possible to fade down and appear much less muted.
Actually most koi tend to thrive color wise in the wild since the diet that is offered and the sun exposure and such is far greater then usually maintained in a man made pond with a fake diet. We have quite a few prime examples swimming the waters here in Massachusetts into the 20lb class.
End of the day though a koi is simply a genetically defected common carp ( Cyprinus Carpio ) anyway and he pretty much ruled that out so the point is kind moot. Be cool to have another koi to chase though lol.