Great Job man.

I had a similar thing happen to me (rapid yellowing near harvest) to one plant. It looked very similar to yours. It was like the plant knew it was at just about peak harvest time and "shut itself down". It began to just dry out right in it's own pot in the hydro setup. I didn't lose yield, or potency, it was like it was on auto-pilot and began to die. The others didn't really do that and were no different in the final product. bend a few stems on the yellowing one, it might be drying out, if the stem almost snaps, or seems dry, you can harvest.