It's obviously because the koi are getting an overload of carotene, chlorophyll, Anthocyanins.
I wonder, would koi pellets, with color pigments, be sufficient enough to give the koi ample amounts of this stuff without causing a increase of too much color like you mentioned or a reduction in pigment. althought probably impossible to know for sure, I am curious to know what the exact amount of these pigment chemicals are needed to supply just enough.
Would the variety of the koi greatly depend on the amount of carotene, chlorophyll, etc. is needed?
Would one food good for say kohaku not be so good for ogon, or shiro utsuri?
I hope these are good questions...