the same thing is happening to my bud too! im in wisconsin growing in my attic which has no insulation which is horrible and im too poor for a heater so the temp up there is about the outside temperature. (sorry, im high and i ramble when im high) so i think the culprit is the cold...but the pink has spead to a couple of leaves. ill post some pics soon so you can see. they also have purple stems from the cold but one, at least, is growing fine.