Yea I was told how to breed the yellow out with good barred rocks. But would take probably years to do. I had an APA judge come to the house and said my roo's where winners except for the yellow. Big downside in showing.Why I think its sun related is the #2 cock that was outside had to be separated and I put him in covered barn porch area with no sun but indirect light and when I put him back out he had whitened up but not completely - he has been back out again and has re-yellowed some.
Don't know if thats it but its a observation. All don't do this as much so there are probably some genetics involved also.