Do not exceed 20% wheat in their diet.
After a while you may find that wheat causes the egg yolks to be not so yellow. There is no loss of flavor or nutrition but the color is diminished somewhat because of wheat.
Since much of the wheat that is not suitable for milling into flour is used in industrial egg production those eggs yolks are often not so brightly colored. Everything else in the book; from lack of ACV (with mother), lack or green feed or the inability to drink filthy water out of a puddle, etc, etc is accused of causing pale yolks but really it is wheat in the ration.