Is it just a few branches coming from the base that are green? If that is so, then the rootstock may have sprouted. I would trace those branches down to where they come off the trunk, and cut them off flush, there. If it is a case of the red color gradually fading out of the leaves, heat and drought can do that--most red-leaved varieties turn greener every year, anyway, so I would be surprised if this is actually the first year it was doing that.