Could be a few things. Too much heat. Light too close. Soil got too dry and plant is sucking moisture from fan leaves. Nutrient issue. Ph problem. Pests. Not knowing your setup, feeding/watering schedule etc it's difficult to pinpoint 100%. I have had this issue before later in flower when the plant is sucking the stored nutes out of the leaves coupled with heat/light stress. Although better than blurple, pics under an hps have a yellow hue. Try to get pics with white or natural light to show actual color of plant. I know it's a pain in the ass, but when asking for advice by pics, we need to see what's going on. It's tough to tell sometimes if the plant is yellowing or if it's from the light.
Do u give rhem cal mag when u started flowering? They scream for that. I would say try that. Not much else u could do now. Let them finish. My summer journal Sent from my SM-G935T using Tapatalk
Looks more like its nearing harvest to me. Plants are not suppose to be all green when you harvest. They turn yellow, brown crispy.