Yellow leaves

What are your temperatures in the grow space? Generally speaking, if leaves are curling upwards it usually points more towards nutrient burn/toxicity from overfeeding. It could also be related to heat stress. While humdity is important, I grew a few plants in a closet for 8 weeks before moving them outdoors and the humidty hovered between 24%-45%, so I personally don't think that's the issue, unless temperatures are way too high. The yellowing at this early of a stage could also mean deficiency and the plant needs more food. At this point, the plant needs more Nitrogen to promote vegetative growth, provide food through photosynthesis, and help give it that healthy green color. Without enough of it, leaves will fade to yellow like in your photo. Best course of action is probably to give a good watering to flush out potential excess nutrients in the soil, and when you start to feed again, maybe dilute the solution with more water, or start with only a quarter (1/4) dose of the recommended feeding. Then as you monitor and the plant progresses, you can increase the nutrients accordingly. When it comes to nutrients and fertilziers, my rule of thumb is always "less is more". Without more information, it's hard to say exactly. Hope this helps. Cheers!