Stems turning purple

Sorry i dont have any pictures. I planted my 4 girls outside one week ago and there looking great. There about 2 feet tall and the leaves are nice and green, but the stems on three of them are turning purpleish. I dont know what kind of seed they are, most likly all the plants are different. And i know some can turn purple, but is this any sign of a problem???.....Im not really to conserned because they look great besides that, but if anyone thinks this might be bad please tell me what i can do....

thanks

purple is a normal color for weed, probablyjust the strain or cold nights, i planted 10 k2 seeds 4 years back and 2 had bright purple stems by fall, as the buds developed they too were very purple and smelled very much like berrys or grapes. i have never deliberatly grown a purple strain but they are out there, cold nights in the fall will trigger many strains to turn at least a little purple.

thanks beaner