Why are some eggs brown and others white?

All chicken eggshells are made mostly of calcium carbonate, which is naturally white. Hens of breeds that lay brown eggs deposit a pigment called protoporphyrin IX on the shell in the last hours before laying. Blue-green eggs, laid by breeds such as the Araucana, contain a pigment called biliverdin that runs through the whole shell. A rough guide is that hens with white earlobes tend to lay white eggs, while those with red earlobes tend to lay brown ones, although there are exceptions. Nutritionally, brown and white eggs are the same; the hen's diet matters far more than shell colour.