You say you aren't overwatering, but you might consider underwatering. When you water, are you really completely soaking the mix in the pot? Hopefully you aren't letting the whole pot dry out. Yellow and shriveling leaves will result from underwatering. Purplish might be phosphorus deficiency, but with your fertilization, that's unlikely, though half-strength 2-2-2 sure isn't much.

On the watering, did you poke drainage holes in the bottom of the cups?

@vgkg Z-7 Va yes i did

@daninthedirt (USDA 9a, HZ9, CentTX, Sunset z30, Cfa) yeah i was considering that too but it happens with all my tomato plants whether theyre watered or not

Doesn't temperature affect phosphorus uptake?

What's the temperature like?

When all tomatoes I have grown dry out they wilt often a LOT. One plant looked dead when I got back from vacation. It came back. My point is underwatering is very obvious. I have this happen a lot and apparently at random. I have a recent post about the same issue

I'm guessing that they are purple because they are cold. That is NOT a problem. I grow my seedlings in my 50F basement and the cool temps keep them nice and stocky. They soon turn green when they get more warmth.