Why are my seedlings turning yellow?

Coco coir is extremely poor on nutrients and it even locks them to a degree. They need fertigation. Use nitrogen rich fertilizer during the vegetative phase. You can use slow relese granular fertilizer or even better, liquid fertilizer. Flush the substrate between fertilizations and water with normal water. Good luck!

WIthout a decent picture or more information on your medium, there is not much we can do to help you. From what i see, you are not watering correctly, please saturate your whole medium and keep it saturated. Roots will not venture in dry places, since you water very localy and your close to the side of pot, your medium dries out too fast. Your plant is struggling from under watering. Crank your humid up to the 65-70% range, Saturate your whole medium, check your Par and adjust correctly. 200-400 Par – for new seeds or clones, as well as parent plants. 400-600 Par – for plants in vegetative growth. <-- 600-900 Par – for plants in the flowering stage. Next time, center your plant in your pot. If you are running multiple plants in a pot (Same strain) keep in mind you will have to water and feed alot more. If you are running multiple plants in a pot (Different strains) keep in mind that different strains have different requierments. It is not advised for beginner growers to attempt and grow several strains at once. Keeping these things in considiration, your grow should have a good start. cheers!

Not a humidity problem. Looks more like burning from a too hot soil mix or from over feeding to me. Post some pictures taken in normal white light so we can actually see some details and give better help.

No, it's not the humidity. I can handle 30% without a problem. Maybe it's the watering that's off? I see the plants are in larger pots? How often are they being watered? And 400w? How far was the distance to the seedling? If you only had 20 or 30cm, that would be a bit too close. Are you using the HPS from the start? That's not a problem, but it's unusual. I guess you might be watering incorrectly... I need more info.

As a guess I'd say it your humidity causing you issues. Ideally I'd aim for 60% rh with temps of 26c

Need a diary for this grow - and better photos... take some in natural light so we can see their real color! Offhand, I'd say they are wanting a little bit of food ... don't know what you're growing (auto? photo?) but start them off slowly - 1/2 strength or even 1/4 strength nutes... You'll also need to increase your humidity - get it into the 60s at least... Good luck... please start a diary!