This might sound weird, but ever since I recently switched back to fluoros for veg, I got purple stems on all three strains.

No fert, no nothing. When i had them under HPS and MH, they were nice and green, so i figure it has to be the lights. Also, the leaves are alot darker green, wider, and much healthier looking. They're only a couple weeks old right now, so i don't know if they'll grow out of it, but i can tell you that they are healthy as hell. It's not a problem, in my case, so i hope your's is similar.