Why are my plants leaves so bright green/yellow?

Too much P and K seems most likely as the top growth looks mostly affected.

It could be several things. First, it might be nothing-it's hard for me to see from the pic, but new growth is often light green. Second, have you changed your lighting at all? Moving a light closer or installing a higher intensity light could be giving them some light burn, this commonly affects the leaves closest to the light first. Lastly, it could be a cal/mag issue. I would try giving them a dose of calmag and see if they begin to improve. You could also make a foliar spray of epsom salts to see if it is just magnesium causing it. Good luck!!