i see this a fair bit, here in the UK during the winter months the incoming air is very cold, my Jamaican grape last winter turned very dark red purple /
during the summer months this red/purple colour change was much less pronounced
during the winter if left unheated the lights off temps can be between 55-60f in my grow room
running the temps at 85f is the optimum temp in my grow room, they drink a hell of a lot more and need feeding more, than at 75f, as a result of more heat they grow faster, more noticeable in veg they seem to explode with the hotter temps
you wouldn't need to keep it cold until the last 2 weeks or so of flowering for it to turn red/purple very fast .. so get the benefit of normal running temps and heat , and just run it cold for a few weeks at the end, cold icy air blowing on them will if they are predisposed to the colour change show quickly
i see a video recently of someone using a huge amount of dry ice at the end of the grow, it seemed pretty pointless as a method of enriching co2 ..
although someone said, this is a possible way of bringing out colours due to the cold atmosphere the dry ice creates .. dunno if there is any truth in that
