Red/purple stem with yellowing leaves

Well, not that i am an expert, but i have seen this a few times. Most likely they are getting too much light/intensity. Try take a look on some of the hidden (covered ) stems and see if they are still green. If so, you have to raise your lights. Another reason could simply be the genetics or early signs of a phosphorous deficiency. PH can also play a part, as nute lock-outs can cause this reaction too. A few other things can cause it too, so in order to help more appropiate, we need more info on stuff like, PH / Ec or PPM, temps at night and such. Are they stunted in growth too or do they keep growing.? Do you measure run-off.?