actually I do have one, I've had several strains do purple stems, sometimes it's completely natural, I've heard sometimes it's caused by N deficiency..
so yeah, I have multiple opinions, one: you're a noob and you would know the purple stem genetic trait as well as the N deficiency possibility if you had read even a little more than 5 mins...
oh that's two opinions in one sentence my bad.
either way... if you've given them enough N, your PH is balanced, you have nothing to worry about as it's natural to the strain.
observe:
http://i162.photobucket.com/albums/t267/poplars/bud 10/Picture435.jpg
chitowns sour kush grows with hella deep purple stems, I've only seen ONE phenotype that grows with no purple stems under perfectly satisfied conditions.