Some people seem to think the different colorations on stems are from drastic temperature changes, such as getting too cold at night. I have seen it happen where temperatures are fairly constant, and when they change by 20 degrees or more from lights on and lights off. Some strains just produce the different colorations also. Usually about 1/3 of my plants have this purple coloring in the stems and they are all together, so in my thoughts, the temps may or may not play a role. Either way, it's never been a worry to me and has no effect, good or bad on the end product.