It is like leg color in yellow skinned breeds. The same yellow for leg color also colors the yolk. Hens that lay a lot of eggs will lose the yellow in their legs. The yellow will come back when they molt. You can make the egg yolks a darker color by getting them to eat marigold extract and or corn. That increases the yellow in their system.
The bile salt that makes the egg blue will be come depleted in hens that lay a lot of eggs. The color will come back when the hen stops laying egg during a molt.
Edited by ronott1 - 6/2/16 at 5:35pm