According to Michigan State University Extension, egg color is determined by the genetics of the hens. The breed of the hen will indicate what color eggs she will produce. For example, Leghorn chickens lay white eggs while Orpington’s lay brown eggs and Ameraucana produce blue eggs. An Olive Egger, a chicken that lays olive green eggs, is the product of a cross between a hen and rooster that are from a brown egg and a blue egg laying breed. An interesting tip is to look at the chicken’s ear lobes; typically those with white ear lobes produce white eggs.
TIL: the color of a chicken’s egg is highly correlated with the color of its ear!