Take a look at my cotyledons!

This pic is from a plant on Day 4. I think it looks pretty small for Day 4. Temps are pretty constant at 70 degrees F, humidity is around 70%, it's in rockwool, and I've given it a small-decent amount of ph-ed water.

What could be the cause of this yellowing?

Thanks for the advice.

may just be the pic but i dont see any signs of problems here. ive learnt that seedlings can show multiple shades of greens and yellows and come out looking great. be patient and dont over water. just keep moist. check your placement of lights. make sure your not burning them.

I dont see any yellowing, and the only major problem that would be visible this early
would be over watering, or heat stress and you didnt over water, and i dont see any burns

what about your ph?
what kind of soil?