Jan I have heard (via internet) from Koi hobbyists that if the Koi grows fast, then it will achieve it's color potential later. Vice versa for Koi that grow slow. 1 of my older Koi is very faded, while 2 have kept their color, with one being more beautiful then he has ever been. So I suspect genetics play the greatest role, if the Koi are kept in good conditions.