well I think you need to check your ph. I do not think stems that purple are a good sign, in fact, in my experience (which is not a ton, but I've had enough problems to know a bit) purple stems are a sign of a nutrient imbalance. Anyway if you aren't going to check your ph, then the best advice I can give you is to flush them with ph'd water (6.8 would be good), then give them a few days to dry out, and then give some nutes (err on the low side). your healthy leaves are very green, so the plant has not been starved, and can take a good flush. The problem with not knowing your ph is that certain nutrients can get locked out if it is too high or low. If you are having problems, knowing the ph is a great tool to help you diagnose.
Good Luck!