Stems changing color to red

if it's just the petioles (stem refers to branches/trunk, but petioles are for leaves), i wouldn't worry about it. Especially if it is mostly just the petioles exposed more to light. you can look up a leaf symptom chart and it'll say low P or something is a potential cause, but this shit happens under an LED. So, if you don't have any coinciding visual effects, it probably has nothing to do with a deficiency. plant looks healthy. probably nothing REd and purple can be genetics too. IF you see read/purple streaks in stems (not petioles) this can be low S... again, you usually need to cross-reference more than 1 visual clue to accurately diagnose things, and even then it can come down to a best-guess. Visual symptoms are not discrete. Relying soley on leaf symptoms leads to false-positives and mistakes. Use all the info avialable.