I've seen that coloration before - the toxic, spreading malaise on the leaves, I mean. I can tell you what I did to cause it. I had been testing some composted chicken manure on several plants. They grew like crazy during veg and developed the symptoms once they went into flower. I put too much of the chicken poop on them and probably poisoned one with too much nitrogen or something.
If you've already heard or tried this advice, apologies. I would flush the hell out of it, give it a couple of days and feed nutes at 1/4 strength until it recovers. Worry about a happy plant and not so much about yield.
My way of flushing was to take it outside and use the water hose for 20 minutes or so to run a lot of water through it. I've also stuck plants in the shower to flush. Any way you can give it a really good flush.
I have overfed too many plants to count and learned that following the directions on the bottle is too much for half the strains I've grown.
