my powerplant has been known to do the very same thing! alot of the petioles are purple too (petiole=stem on leaf) along w/ some of the outlined leaves. it might be a trait, or it might be the temp difference - i know purple is commonly sourced from being cold @ night during flower - but ive seen purple petioles / outlines during veg too. the petioles are not a sign of deficiency cus my plants are as healthy as they could ever possibly be, but the outlined leaves i dunno. id like to know exactly why it turns purple though.