I read somethin a while back about cannabis stems and stalks turning purple/reddish to reflect the light it doesn't need on those areas. Not saying that's it and you have no deficiency but I thought it was worth a thought, maybe some further research
LEDs convert lots of sugars but also causes translation problems. I'm no scientist but it's obvious.
Practically every led grow I've seen, you can spot nitrogen puddles sitting mid leaf. That's related to calcium flow. The sugars are transported with phosphorus and magnesium.
Anyone who wants to argue its not sugars that make plants purple, show me your phloem! It's a specific sugar (and probably aminos) that's abundant in purps.
Purple leaf stems are normal for many strains. I actually like seeing purple stripes usually it results in plants that color up nicely during cold weather.