Why are my plants turning purple?

I'm growing a bunch of autoflowers, 3 la diva's, 2 ez ryders, 2 afgan kush ryders. I know the la diva's have a purple pheno, and out of the 3, two are turning purple now. But I havent heard of purple ez ryders, or afgans, but one of each of them is also turning purple now. It started on the edges of the upper leaves and is spreading inward. A nice dark violet. I'm growing under 400 hps, in soil, temps always mid to upper 70's, and on day 49 from seed. Could it be something with my enviroment, or what?
P.S. I'm not complaining.

are you watering with warm or cold water? because I have heard cold water will turn a plant purple. IF not that, then maybe the nutes r turning them.