This a mother under a V1 3000k 80H 24/0. Stems always turn purple under high light and slow growth. I've had it happen on revegs and on 12 week 13hr on flower cycle. Right now, its got nute burn because its gone into some slow growth after i trimmed, theres no deficiency or cold though, it doesnt typically purple either. The problems we have is we never see a plant from the top usually from the side. From a top angle, anything thats getting light usually get covered by new growth. I got purple under some blurples too. I had some large plants under a HPS and a MH and used a blurple to supplement some side light, everywhere it hit, it turned the stems purple. Half of its genetics and half of it depends on light. Some people blame cold or deficiencys, those also create slow growth, slow growth leads to old growth not being able to deal with high light without new growth coming in to protect it. Leaves are more rare to purple because in high light they tend to bleach, yellow crisp up and fall off, theyre just too delicate. Some say that the pigments are created by hormones, and that once those purple hormones start they tend to keep going a while, so if you get purple stems in veg, no matter what theres a decent chance you can get some purple bud. My theory is that if you were to put a little UVA in with 301B, it could kick in those hormonee and it would purple almost anything, i think HLGs on the same line if thinking. It all comes down to the ratios, id imagine thered be so much heat and UV on HID it would fry most stuff before it could purple. My 600W MH gave me a few crunchy corncob looking colas.