Hi Silkie,
I'm thinking food doesn't play a part but . . . . I see so many things where food is very important that I could be way off. In general, quality food makes for a really healthy body, chicken and human. HOw the body uses it, is basic health. I see egg coloring as a fucntion of genetic coding.
I have a hen that produces a soft olive green shade over the entire egg, then speckles it with splashed of red-brown paint. Always looks exactly the same. I know her egg; no other egg like it. Wish I could have more hens producing that coloring, but since all the hens get the same feed and live in the same yard-- me thinks genetics is the difference.
I hope others will chime in with their thoughts.