Purple and reddish colours show up on some sativa lines when night temperatures drop below fifteen degrees in late flower. It is purely cosmetic and comes down to anthocyanin expression in certain genetics, so a plant that stays green in a warm tent is not doing anything wrong. The colour genetics discussion on https://hazesativaseeds.com (https://hazesativaseeds.com) lists which of their lines carry the trait and warns that pushing temperatures too low chasing colour slows the finish on already long strains. A few degrees of drop is fine, a cold room is not.