Just off hand, some strains turn purple because of my A/C being cold. Some turn because just not enough nitrogen. Then some strains the PH may not be where they like it and not uptaking nutrient.

The trouble shooting form will get you more help.

If you have A/C and it's around 70 degrees and everything has been going alright and they are still looking good and growing then it's the temps. Nothing to worry about.