good response but what ever it is, is all of them in coco, which makes me think, either it is not ph, OR his meter is out of calibration.. He should calibrate meter immediately to erase this from the equation. So next is roots and feed. I say, pull one or 2 of the plants, (worst ones), and take a pic of roots, and we can start to narrow down the cause..
If roots are good, then switch feed, to something simple for a week, and see if improvements in color happen.....
But all this, is for nothing if you are being lazy and going more than 24 hrs without feeding... Ive seen this before! Its usually from a lack of watering on a timely manner, and to 15% runoff, with a 1.0 ec...
Anyway, i would run 3x the pot size of 1ec nutrient solution thru them, let them dry 36 hrs, and start normal feeding at 1. ec to 15% runoff... I would also pull 1 plant a week from the pot and look at the coco, and roots to see how they look... I would also use 5ml per gal hydrogaurd once a week in your feed. I would also give light feeding of some kind of sweetener, to feed the hydrogaurd bacillus bacteria... And a root+foliar pack would be a definite plus!