The color of an egg shell is determined by the pigments, or lack of, deposited as the egg develops. The pigments deposited are determined by genetics. All eggs start out as white. It takes 26 hours for an egg to travel down the hen's oviduct. It's during the last 20 hours that the shell is formed. For white eggs, no pigments are deposited on the egg during this process. For brown eggs, the pigment protoporphyrin is deposited on the egg late in the process. The brown does not penetrate the shell so the inside of the shell remains white. There are many shades of brown eggs. Some very light browns almost look pinkish. Blue and green eggs are formed when the pigment oocyanin is deposited eariler in the process and permeates the shell, resulting in an inner shell color that is also blue or green. So how do you know what color eggs a hen will lay? A good rule of thumb is to check the hen's ear lobe color. If it's white, she will lay white eggs. If it's red or brown, she will lay brown eggs. There are a few breeds with red ear lobes which are an exception to this rule. Then there are the Araucana and Ameraucana hens which lay the blue-green eggs. When a blue/green egg layer is crossed with a brown egg layer, the result is an olive color egg. These hens are called Olive Eggers. The blue shell develops with brown pigments added towards the end of development process. Another tidbit of information for you... An individual hen typically lays eggs of the same color and hue.