Its hard to say exactly, anyway you can snap a few with the hps off? from that pic it looks like they are yellowing near the bottom, seeing some purple stems, and it's either lack of humidity or under fed up top. what's your max temps in your grow space right now?
I noticed you listed Coco soil as your grow medium. if that's what your still using it could be that the coco is just pulling some of the feeds out. If you haven't started feeding them yet, that may be an option too. They also look like they should be near ready for potting up (atleast from what i think the size of that pot is from the pic). Give a good look at the root ball and see. it could be they are just rooting very fast and sucking the soil dry of feeds fast too. a bigger pot and new soil should help some. Regardless your lower leaf stems look purple early on, when the soil should have been at it's best, so it's tough to say mate lastly maybe ph problems in the soil, hope something in this helps you solve it

*edit* fat fingers