I had a nitrogen diff. that started about three weeks into flower (a bit early) I gave them some nitrate for a few weeks (half the reccomended dose) and things perked up very nicely. Growth also seemed to pick up. Now I am 7 weeks into 12.12 and I won't be giving them any extra N, though my blooming nutrients do have some in it. The yellowing will continue from here, very natural for it to happen. However, if you feel like it's happening too early or the yellowing is causes any stunting to the growth, shoot em up with Nitrogen! My babies def. benefited from doing so. You can always use half the reccomended dose if you feel like you would be giving it too much. Play around with it, and your babies will have a beatiful green lush color within a week of givng em additional N, mine did!