Yayah
Songster
There are only two colors for eggs shells. Blue egg shells are blue inside and out. White egg shells are white inside and out. Brown eggs are white eggs with a brown coating. They are brown outside, but white on the inside. Green eggs are really blue eggs with a brown coating on the outside making them look green. Different brown 'coating' genes affect how much coating gets applied creating the different shades of brown and green eggs.
Thanks for the succinct explanation! I'd never bothered looking it up, but that's very interesting.