Purple stems could be genetics,
Temps below 60 or magnesium and/or phosphorus deficiency. Make sure your ph is in the proper range or you could get lock out issues, preventing uptake of magnesium/phosphorus look into adding a
calmag or just extra magnesium. Being in veg? I doubt it’s a phosphorus deficiency. Always ph your water
If you can provide more details on the grow would be helpful. What are you growing in, light info, temps humidity, feeding schedule, what type of water and how often, etc…