MORD is correct, the buds are sucking up all the nitrogen at this point to grow more. You can add more nitrogen to keep the fan leaves green, but I've never heard or read that this provides any benefit. Just keeps the plant looking green.
I've got a Blueberry grow at 6 weeks flower and the fan leaves are turning from purple to yellow. I'm not too concerned, cause the buds are growing bigger and getting frostier.