Usually red/purple stems and yellowing leaves are the result of a nitrogen deficiency, and can happen to any strain. Cold can cause this to happen as well but normally starts turning the leaves red/purple, even black, along with the stem... The most unlikely thing to turn your plants purple is not enough light, this simulates a nitrogen deficiency by not allowing photosynthesis to happen at a normal rate, leaving the available nitrogen unused, and the plant yellow, purple and sickly looking. Hope this helps Beansly!