From my understanding, purple in cannabis plants is one of two things : genetic or from a nutrient deficiency. Yours is most likely due to a nutrient deficiency due to the fact that it is the actual stem that is purple and not the buds (I could be wrong, some cannabis plants may show some purple stems). Colder temperatures (especially a light frost) will also induce a genetically predispositioned cannabis plant to become more purple. Just my one and a half cents.:jointsmile: I would post this question more logically in the plant problems section