Which strain are you growing? Is it an Indica? I've got some clones from a dispensary and most of the stalks were streaked with purple. They were a bit nute deficient and began feeding them with House and Garden's A and B base with Root Excellerator. They are now showing a slight sign of over fert as the tips are browning a tad, the purple stems are constant, and they are growing like mad and healthy otherwise. Plenty of new root growth too