Why are tese turning yellow?

its only on 1 sprout

its at the corner of your tray and the farthest from the light (assuming you treat them all the exact same way). move them around in your tray daily. shifting them ensures a more even absorption of light. id say POSSIBLE nitrogen def. but idk how old they are (look about 7-10 days old from seed). hope that helps. who knows, i could be off base as fuck, but thats what is most probable. so move it to the center and see if it changes.

Agreed, it needs more light. The long, spindly stalk is a dead give away.