Without knowing all the details, I would guess one of the following:
1) improper pH causing nute deficiency
2) over feeding (nute burn) <- I am going with this one for now
3) Using HID lights after foliar feeding causing burn spots
That's what I thought at first [nut burn]. One of my friends was the culprit for these problems as he didn't follow my directions AT ALL. Anyhow, I immediately flushed the plant and for about 1.5 weeks and it didn't get better. In fact, newer leaves that were fine after the initial problem began to have problems.
It's soil. The temperature is fine during the day (HPS lamps providing warmth) but at night it probably gets to around 60ºF, probably causing the purple leaves.
I made a new solution of nutrients to see if maybe if one of them got locked out. I'll see what happens tomorrow.