I have a question: are all the green egg layers having blue and brown egg genes? There is a PURE breed Green-egg-layer chicken (Yes, that's their Chinese name.) that exists thousands of years and all of them lay green eggs. If they have both blue and brown egg genes, then at least 25% of them should lay brown eggs, right? Here is how they look like:
http://www.wanwan.com.tw/viewthread.php?tid=3201&extra=page=1
(Their eggs are the very uniform green color. If you scroll down see the pile of brown-green eggs, that's from British EEs for comparison.)
By the way, does any one know if I can find those chickens in USA? It would be fun to have one.
No, if they are homozygous for both blue and brown, they will lay green eggs and will breed 100% true to that.
There are many (at least 7, from what I recall reading) genes that influence the amount of brown pigment. Some interact and probably some are independent. Some increase and some decrease the amount of pigment deposited on the egg. It's all very confusing and could make producing a true breeding green egg layer non-trivial, but it's certainly possible to do. Isbars are an example of a breed that is true breeding for green eggs. They are not common, but I've seen them advertised on Craislist at least once in the Reading area, so definitely possible to obtain.