It's even cooler when you know how the green and blue work. There are only two true egg shell colors, white and blue. A bird that has at least one copy of the blue egg gene will lay blue shelled eggs because blue is dominant over white. However, if the bird also has genes for brown coating, then the brown coating over the blue egg shell will give a green color to the egg. Birds that lay brown eggs have a brown coating over a white shell base color.
There are many genes for brown coating, which is why there are so many different shades of brown eggs. That's also why you get so many shades of green in EE eggs.
I love my EEs. I like my Ameraucanas, too, but I find the EEs so much more interesting. I know what color eggs my Ameracuanas will lay as soon as they are hatched, but finding out what color green or blue egg my EEs lay is like Christmas.