purple spots on my stems?

So my plants are about 2 months old and I just pruned all of them and they are doing well sprouting new stems. I'm pressed to force them to flower because I will be moving at the end of the summer, so I've been gradually cutting back the light until I get to 12. Right now, it's at 15/9. But, I've noticed purple spots on the top stems of two plants. Not the entire stem, but a good portion. It worries me some, but the plant doesn't seem any less healthy because of it. What could it be?

are you sure its purple, anything purple is o.k. at 2months there perfect for budding start them on 12/12 light cycle with total dark at night. enjoy the smoke!!!!