Yellow seedling help

50% may ore may not be fine.. RH alone is not what matters. RH + temp is what matters, aka VPD. VPD requires leaf surface temps, which are going to be 2C lower than atmospheric readigns, give or take. Go image search a "VPD table" and follow the suggestions. There's a wide range that should work well... 1-1.5 VPD shouldn't cause many problems. Early vege and clones may be better with a sub-1 VPD. There is a wild range of suggestions out there, so take them as a grain of salt. Observe plants and react to how they actually grow to fine-tune it without some pre-ordained number creating a bias.

i'd hold off on reacting. It's too young and need to see more to confidently diagnose. if that is soil, probably doesn't need fertilizer. could be balance of what came in the soil as far as any resulting issue. If soiless, better start fertilizing immediately, as that is necessary for that grow method. use an appropriately sized pot. It is difficult to water a tiny plant in a big pot, properly. It elevates probability of rootzone issues and poor watering habits, which could be causing what you see, too. if you aren't saturating all the way down to the bottom, you are doing it wrong. when youhave a tiny plant, you do that in a smaller diameter around the plant so that the substrate doesn't remain wet for weeks on end as it cannot drink it fast enough to avoid risks of stagnating water. even if it is an autoflower, you should pot up as necessary. anyone that says potting up causest 'transplant shock' is lying to themselves about it.. total denial of reality. I've done it 400 times? or thereabouts and yet to see 1 plant get shocked... no 1 in 6 years so far from potting up. It's an absolute myth that you shouldn't treat an autoflower like nearly every other damn plant in existence should be treated, lol. it's a plant, not some mystical, magical lifeform. just a simple soft-stemmed flowering plant.

Your humidity is wrong, if you have 75 far ( i would recomend in veg stage even 80 ) , your humidity should be at least 60-70% . Leaves has long stems, so guess light is too weak. Maybe direct wind is blowing on her too, oscillating fan is a must in weed grows. Hang wet fat bath towels daily to raise your humidity. Plant will recover as it grows , but you wasted a week or maybe more from optimal grow rate. Start was nice ... I liked prohp soil mix, just it was very potent, so no feeding for 3-4 weeks, unless its micros or root stimulants. Your watering regime is wrong. Whatever others will say - pro mix hp is still soil based medium, so you must have wet/dry cycle, watering every second day is depleting roots from oxygen. Normall schedule is 3-5 days, depends on plant, pot, humidity and temps. Start lifting pots to know when to add water. With small plants quantity is a bit tricky, but 400 ml is what i would be giving for such size. 6.2 ph is on lower side, try to bump to 6.3-6.5 .