I've got some bagseed that I started a few months ago and cloned for my new hydro setup that is doing that now too. I'm real new to this so I did a search and found this thread. Is it a disease or something normal? It started first on my mothers and now my clones started doing it too. Since it is bagseed I don't know much about its genetics. I don't even remember what bag I found it in. The mothers just started to show their sex, if that matters.
I tend to agree with bong, I have had plenty of plants with some purple to lots of purple on the stems, the leaves really tell you, if you have really slow growth or something, then perhaps I would be concerned. The only thing I have really heard is a potential phosphate deficiency or lock out.