The EE will always lay the same color. It is like her signature. The exact shade of light green may vary though. A recent post on BYC told about how strong direct sunlight can fade the shade of the egg shell...and I think that my EE who lays light green has gotten a bit lighter lately.
Also chickens will lay eggs in a clutch, if part of the green is from her brown heritage, then the color could lighten. You will have to be looking for the color change to see it---and perhaps even blow out some eggs, keep the shells and compare over the months. She won't switch from green to blue, to brown to white though.....