Purple stems mean very little usually, people always freak out about purple stems, DON'T SWEAT IT!!! your leaves will tell you when there's a problem and lots of breeds grow with purple stems, WW, AK-47, White Rhino, Nebula (both strains), Somango, Chitralah, Bhuddas Sister, S.A.G.E., Black Domina, etc., all have purple stems.
it can also be caused by low night temps, don't worry, it'll be all right......
I forgot, also Biddy Early and Warlock do the purp stem thing.....