Top of plant turning yellow and sagging

The top 1/3 of one of my plants is turning yellow. I was reading it could be Sulfur? Not sure how I would add that. Then I was thinking cal/mag maybe? I haven't used it on any plants yet. Maybe a milk foliar spray for calcium

It's the plant with most/best buds at this point and want to keep it as healthy as possible
2 other plants same strain,nutes,soil look ok, it's this plant specific
4/5 week flower
1:2:1 ferts EC 1.8
not overwatered

Any ideas on what I can do?

Plant that size for a pot that size, I would be focused on feeding before anything else. Your other two plants you say are OK, so lets for now assume, that she just needs a bit more neuts than the others - perhaps in the form of a good tea - no harm in starting there, and see if she responds well before anything else. If she is the biggest and best, then she likely feeds more than the other 2. Overall she looks good, just a bit yellow and hungry.

Nice compost tea should have some nice effects.

So I have another plant same size that has some yellowing on the top now. not as bad as the one in the pic So I did a test. That one I added 2.2 EC ferts. 1:2:1 . The one in the picture I did a water/slash flush with 3 gallon of straight water. Neither changed after 3 days
I'm stumped.

It's been cooler that last few days, mid 60's/ low 50's high 40's at night
I'm wondering if the cold has anything to do with it?
It's supposed to warm up to mid/low 70's for at least the next 7 days. I'm wondering if that will chance anything?

A couple leaves out of the whole plant had some purple at the tips and edges. It was literally 2 or 3 leaf's out of thousands Oddly on the two 8 feet plants they were in the same place about 6 inches from the top. I'm thinking it was the cold over a P deficiency but no way to know for sure
probably 6 weeks in flower right now.