Smoke, I have purposely starved my plants of nitrogen but not of calmag. As a result, no purpling just yellowing of the leaves, starting from the older leaves first.
How I always fix purple or red, even if it is just streaks is to correct temps of room, be sure it doesn't get too cold, as when it is cold the plant has a harder time uptaking K and Mag.
If the temp is good and no salt build up as in using my coco system, then the only other thing to fix, if not a disease is PH.