I have been reading a lot about this problem of late yellowing. A lot of people have been adding their veg nutes with high nitrogen later in flower once to fix this problem.
I have been reading a lot about it as well. Seems a lot of people are having the same prob. My WW's started early, 2 weeks into flowering. I added veg nutes but it never recovered.
I think the leaves should be healthy and green until the flush starts or you drop the nutes. Every flower, plant, bush or tree I have ever grown was the healthiest, best looking and grew the fastest when the leaves were healthy. So imo it should be the same with weed.
This is what I am trying on my new grow to combat the yellowing. This time for the first flower feed I just added 1/2 strength flower to the veg. Next feed will be full both flower and veg. I will slowly cut back the veg nutes as I see how they do. I dont know if this works but I am going to try.