I dunno, but to me that looks like a natural reddening that comes from either your strain or the temperature or a combination of both... I've seen that several times. Sheesh, look up Soma A+/ Rock Bud. As the chlorophyll breaks down (what usually causes yellowing), some strains start to show the OTHER -phylls, one of which is a beautiful autumn red. Since your leaves are otherwise glossy and nice, with no necrotic patches, I have to say you are seeing that phenomenon. No worries.