Hey Growmie,
Actually neither of these will cause red/purple stems, but a Magnesium deficiency can cause purple stems. A good cannabis-friendly nutrient system has a significant amount of magnesium, but if you’re using very soft water, or RO water, you may need to add a Cal-Mag to your nutrient regimen. Also, Temperature can play a part in this too this – Heat or especially cold can trigger red stems in some places. Big temperature fluctuations can also do it even if the temperature is in a good range. Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.