Seedlings - New Purple leaves

Hi, I have about 9 seedlings right now, they are on their 2nd and 3rd set of leaves under CFL's in hydro. They are made up of 3 different strains, so I ruled out genetics. All of the plants are starting to grow purple leaves as of the last 24 hours. I am using Dutchmaster Grow A&B only PH 6.0, in rockwool and hydroton. My PPM is around 450-600. I use distilled water as my base. Anyone know what causes purple leaves? Thanks everyone

what is the air temp at night?/