Heh, what autoflowa said . . about the lack of N. This happens naturally during flowering. It could be you didn't get quite enough N into your plant to last thru the flowering process. From what I've read on here, there's not a complete agreement about adding more N at this stage. I think some folks will say to let it finish without adding a dose of N. Others will say to give it a dose, without overdoing it, to get it through the remainder of the flowering stage.
Myself, I have a plant in flowering and I just gave it a boost of N, probably for the last time. But, I suspect it still has over a month of flowering to go. I had/have yellowing leaves too.
or what could be the problem they were green....
