m0useanswered grow question 3 years ago Some plants just get that when when exposed to LED's and if its in their genetics.
Their are deficiencies that are associated with phosphorus and red/purple stems but they tend to also be showing on the leafs.
Your plants are looking Healthy and I would not worry about this at all.
My G13 Blue Cindy is also doing this under my grow and she is looking healthy as well.
Best Of Luck!