DrFever is spot on! My plants are always nice and green until I switch to flower where things start to go yellow (2nd week). This happens mostly to my pure indicas, they just start shedding leaves even if I add a bit of N. Some will require more N in flower than others and using flower nutes will have an antagonistic effect on N further augmenting the yellowing. Also molasses tend to interfere with N availability from what I read. I always try to keep em green till the end (hopefully next time I will manage), I guess it is time to get some cal-mag as well.
very nice looking plants you have, hope they remain so happy till the end!