befe76a6_al_Approach___The_Poultry_Site__Control
[Cause] Oxidising agents or pigment antagonists in the feed [Control] 
The addition of a recommended antioxidant to feed will help preserve yolk pigments. Either do not include high levels of barley or triticale in the diet, or, if they are included, adjust pigment levels to higher than normal concentrations.
Pale yolks - Optimum Egg Quality: A Practical Approach - The Poultry Site
Control