when I see a sudden change in the appearance of a plant in soil...I look first at the ph,then at the age of the plant....plants beginning to show sex sometimes enter a phase where they eat and eat and drink like crazy,just before the growth spurt. when this happens you need to play catch-up with a 1/4 strength feeding and I like to spray a 10 part water/1 part ammonia foliar feed for a N boost.
If you decide to try this method....remove the plants from the grow area 1 hour after the lights come on...mist the leaves underside and topside....allow them to completely air dry...don't put wet leaf under hps..it will spot burn the leaf....it takes a couple days to notice any benefits....the yellowed leaf will not regain its color...I remove the unsightly leaf,but they will eventually drop off if you don't remove them.
you have some great looking girls going...I hope this slight setback only caused a little anxiety and not alot of stress on the plants..