A green egg is a blue egg shell with a brown layer applied over it. If the blue shell color stays consistent during the laying cycle, but the brown layer applied over the surface gradually fades to a lighter shade, which is common with brown eggs, then during that laying cycle the egg color will gradually go from more green to more blue. You would also see that type of color change if the brown layer simply fades faster than the blue color in the shell.
With my green eggs, it was more like they went from a darker green, to a lighter green. My green eggs stayed green and my blue eggs stayed blue. I've heard of a couple of people that had eggs go from more green to more blue during the laying season. Long term, I don't think the brown layer would disappear, you would just have seasonal changes to it.
When I originally read this thread, my first thought was to contact the breeder and let them know this. Was this a first time breeding and they honestly had no idea? Do they not know what they're doing? Was it a scam? You know, it's really hard to say, if you don't know the person you got the eggs from. I think their response to this information will tell you a lot about them. If they are a person of good character, that's serious about breeding, they'll want to know this. It will be important information for their breeding program. If it's just someone with a flock of miscellaneous chickens that's selling hatching eggs mostly for fun and a little cash for feed, they might not care. If it's a scammer, well, who knows if you'll even hear back from them.
I agree that it is a beautiful egg.