Dude start to feed them!! Get a spray bottle and folar spray until you get them back to normal. You have a potassium deficiancy big time. No worries though it wont kill them if you take care of it soon.
Potassium def starts on leave margins but causes browning not purpling. This is probably related to phosphorus because he said his ambient temps are 75 which are OK, and not cold stress.
if i am seeing correctly, they are in a party cup. rather than starting to feed, you'd better transplant them asap to a normal sized pot.
Transplanting is your best bet, not because the plant has outgrown the cup, because that soil is garbage and is clearly affecting nutrient availability.
How do you figure it will kill them? 2 things I see from the pics #1 the plant is not big enough to be out growing the cup and the signs to not lend to over watering so the cup is not the issue. #2 he said he had not given the plant any nutrients yet which lends to my first diagnostic of Potassium def. Spray them with nutrients when there are no nutes available anyway would be the way to get them going in the right direction quicker that just feeding them though the soil.
Just because he hasn't fed his plant nutes doesn't mean they aren't there. That little guy doesn't have nearly enough potential to suck that cup of dirt dry of nutrients, ESPECIALLY being in MG. Foilar feeding is not very effective on sick plants (or healthy plants for that matter), the leaves are taking damage and trying to repair themselves, not uptake nutrients. Another common myth is that you spray the tops of the leaves, all that will do is leave salt residue on your plant surface. The stomata are on the bottom of the leaves and that's how it can take in nutrients. These are only open around 72 degrees, which further makes foilar feeding pretty ineffective.
Also, your wasting your time, money, and freedom by growing in mg and feeding with mg. Until you take time, funds, and knowledge and invest it in your grow your not going to have any real success. I'm not bashing on being a beginner, I definitely was once there too. Any experience is definitely better than none. I am bashing on beginners who set expectations for even sub-par results. Under 120w of CFL lighting and horrible soil, environmental, and feeding conditions your plant will stress more than produce. All of that nonsense is nowhere WORTH your value of roaming the Earth free (to a certain extent, if you call America freedom). If you live in a region with decent conditions, you will have 10x better results throwing bagseed outdoors.
Please don't take that as an insult, but I work at a hydroponics store and watch people throw their time and money down the toilet every day. It saddens me when I started with virtually nothing and still produced and progressively got better each time, and thats true to this day. Even using my same genetic over and over, my results have always gotten better.
