Ahh ur comparing seedlings... that doesnt mean nothing if one looks better than the other, there just seedlings. I had 7 seedlings sprout and some were light green some were dark green, one even had 2 deformed leafs (which is one of my best plants now) seriously dont stress over shit like this, save all the stress for when its a mature plant, thats when you will be needing alot of answers to alot of problems. Your seedlings are fine.