I've seen natural undyed antler turn up with reds, purples and olive green tones before, but that is the brightest green I've seen. Was the antler fresh harvested from a deer or a drop that was found?
As I understand it, most color in the interior of antler is due to bacterial growth in the material after it is shed and lays around for a while, but, as with any natural material, you never know.
I will say that it is probably rare, and would command a premium if it's natural, as opposed to dyed.