I have also noticed that the plants grew fast and furious all the way until about 12" away from the light. I cannot move the light any higher and I do not rotate the plants. Leaves that are about 12-14" inches away have been turning purple. I read that purple cannabis is caused by anthocyanin which is mostly photosynthesized in green light according to this chart. Which really has me stumped...
Damn they certainly look good, is that bad for the plant when that happens though? If it is, you should maybe try to LST a bit harder next round, when you LST you can really keep the plants low