Have you read the sticky`s my friend? They have a bank of info on all problems and symptoms you`ll ever see on MJ, purple stems being only a sign of a possible def. Many strains show it as normal genetics.
Does the purpling extend all the way down the plant, main stem included?
If below like 50f, sure. This makes Phosphorus hard for the ladies to draw up.
I have to say those look very healthy to me my friend, what are you feeding them now(NPK), and when last? They look peckish 6-4-3 same as 12-8-6 etc.
Green mojo straight at ya!
MJ can take lots of nutrients as long as they are built up over time, i.e 1/4 to 1/2 to 3/4 etc at each feeding schedule, once to twice a week. Not so much of a worry with Organics, better as you know, and much harder to burn the gals too.
Look into the nutrients section as they have many organic feeding programmes for optimal production of healthy bud
Hmmm, the obvious would be the leaves look great, very healthy, don't worry those plants are fine... you would see other problems already if this was a concern, no need to counsel your fears!!! Nice grow so far...