purple genetics are weak, plants dont grow good in cold whether thats why they turn purple, grow the purple like any other green bud most real purple strains will grow purple on their own, also compost teas and most bloom fertilizer will give ur buds color, if ur gunna drop ur temp do it about 3 weeks prior 2 harvest let room drop 2 40-60 degrees every nite, alsokeep humidity levels in cheak dont want mildew, good luck though