The hen’s genetic makeup decides the color of her eggs, although eggs from a particular hen may vary a bit in color throughout her cycle. Ameraucanas release the pigment oocyanin which creates a blue egg, deposited on the shells as they move through the oviduct, the blue color permeates the egg causing the inside to be blue as well.
Brown egg layers deposit the pigment protoporphyrin towards the end of the egg’s journey through the oviduct, which causes the outer shell to be brown but for the inner shell to remain white.
In an Olive Egger or Easter Egger, the brown pigment is “painted” over the blue shell resulting in a green colored egg. The interior of these eggs is usually blue! So the egg starts off white, then is painted blue (which reaches the interior), then finally the brown color. The darker the brown, as in Marans, the more olive the resulting egg will be.
This entire creation takes around 26 hours. A miracle.