It's impossible (at least in my opinion) to tell how ripe they are without seeing the trichomes coloring. Get yourself a scope and see if you're clear, cloudy, or amber and what percentages of each will help you determine when to cut. The more clear/cloudy heads will give you a more upbeat head high, and the more amber you go the more couchlock the meds will be. Remember as the trichs begin to turn amber, it's because they are degrading and will continue to do so after you've cut her down. Think of it as fruit from the farm, you pick early enough because it will ripen further after harvest.
The yellowing looks like a mag defficency and maybe a little bit of nitrogen defficency but that's expected later on in flowering. If you've just started your flush, there's not much you can do but ride it out and change things next run. Epsom salt is a great cheap source of mag sulfate, I add 1tsp per gallon give or take depending on strain/pheno. Do those buckets have drainage holes in them? If not, I would reccomend drilling some. It's hard to get a good flush without run off again just my opinion. Also as you flush the yellowing is going to get worse and worse, the leaves will begin to droop and some will die, pluck them as they die, and don't worry too much. Again just change your feed next run. Other than that, looks like you'll have some nice meds in a few weeks!
W.C.C.