This is something that I have never worried about but I do seem to remember that the eggshell pigment is the last thing added to the egg before it is laid.
Here try this link.
http://hencam.com/henblog/2012/02/brown-green-blue-white-chicken-egg-color-the-real-story/
Here try this link.
http://hencam.com/henblog/2012/02/brown-green-blue-white-chicken-egg-color-the-real-story/