As far as I'm concerned, when a plant's genetics are 18% THC...I'd rather have more buds to smoke of that percent, rather than leaving my plants in the dark for any more than 12 hours per day

As for the initial question about inducing purple....Cool temps do that 50-60 degrees is probably adequate to achieve that...some strains are more susceptible to it than others, but they'll all turn at a point.