High Matt
A lot of strains have purple stems and it doesn't really mean anything. Your plants look really good overall and with the little bit of tip burn I see in a couple of pics it doesn't look to me like they are needing any more feed at this time.
Sometimes red stems indicate low sulfur or magnesium and I don't see signs of that either but a bit of epsom salts in their water won't hurt and will make sure they get some of both. A tsp/gal is lots for healthy plants not showing signs of deficiencies and every third watering is lots.
What are they growing in and what are you feeding them?
Nice job so far!
