Pics would be really useful, but maybe your plants are a little deficient in something? cal/mag phosphorous? post up some pictures and give as much info as you can about your grow, soil , nutes etc
purpling can be normal depending on the strain or possibly cold temp's but most tend to say its a deficiency of some sort and i tend to agree, when i experienced purpling i was not adding enuff nutrient solution to my water, i increased my nutes an the problem seemed to clear after that, cannot say that this is whats happening to you, nor can i say what nute in particular i was lacking at the time as just raising the ppms did the trick for me,
give as much info as u can and maybe someone here maybe able to help!