Your comment about green being blue with a faint coat of brown.... However the underlying color of the egg can vary somewhat from blue to green blue and perhaps even green. A good way to check the actual shell color is to crack the egg and peel of the inner membrane... The color you see there is the true shell color
I found this discussion on blue and green eggs fascinating and since I knew absolutely nothing about what makes eggs blue, I did a quick Google search and came up with this article:
http://scratchcradle.wordpress.com/2012/07/01/gms1-genetics-of-egg-color/
Here's some of what I found:
A white egg is normal for wild chickens and is represented genetically by "o."
A blue egg, besides containing all the normal minerals of a white egg, also contains oocyanin which is a byproduct of bile. Genetically, blue eggs are represented a "O." I would expect a purebred Ameraucana to be genetically OO. An EE might be Oo. The blue egg is dominant over the wild-type white egg.
As Pips&Peeps said, brown color is a paint that is added at the end of the whole egg production process. The brown can be washed or sanded off the egg since it is just a surface coating unlike the blue which has oocyanin incorporated right into the egg shell itself. There are a lot of genes that influence how much brown pigment (protoporphyrin) is laid down. There are even genes that inhibit the expression of brown pigment. A coating of brown over blue will give some of the green colors, ranging from light green to dark olive green.
I only have two Ameraucanas laying eggs, but I am going to try sanding the eggs to see how the color differs between the sanded and unsanded portion of the same egg. I may try washing them and scrubbing them, too.
I would be interested to know what would happen if the person with the extremely green egg sanded a part of it. Perhaps the green is really just a light coating of brown pigment on top of a blue egg.