I'm growing a self-bred Master Kush/Acupolto Gold hybrid, and I have a few phenotypes, but the strongest in my particular genetic line I work with is by far the purple stems... It's usually genetics that are the culprit, if u run a well-rounded gig, and keep your NPK ratios in check... I think it's got something to do with the strong Indica background myself... As most of the Indica strains, like the crystal-laden purps Ive grown before, kindof tend to exhibit purple in some form or another, whether it's mainly the calyxs, or the stigmas, or the leaves(not due to cold environs), or the purple stem thing... It is pretty common... I used to think it meant something really special, but I can't say that I still see it that way, although it is nice to look at-on a healthy plant, and not on one with a deficiency...