An image was posted from Parade Magazine, in which Rian Johnson teased that there may be another hue of lightsaber in the film besides Kylo's red and Rey's (Anakin/Luke's) blue.
The simplest answer is that it means that Luke uses his green lightsaber.
Or it could mean something else entirely. Maybe something never seen in film before, which would would mean it isn't purple. Maybe something that is indicative of the Grey Force, a concept that might be explored in the film. I doubt it the Darksaber, because that would be weird. Perhaps white?