From what i remember red/purple stems are the beginning of a phosphorous deficiency. I could be wrong though. Its been awhile since My first grow and thats when I had that problem. It could also just be the strain. I'm thinking once you start feeding her, the problem will go away though. Or at least it won't worsen. Maybe somebody will come along with a better memory than I, and confirm or deny this. Either way, if she stays green and looks to be thriving, I wouldn't worry too much about it. Hope this helps. Good luck and happy growing.