If you are past the third week then the yellowing is normal, but next go round you should feed veg nutes high in N until the 10th day of 12-12 in order to give the plant that needed N to carry it throughout the flower cycle IMO. Also I would add some dirt on top, not because the roots will grow into it, but it will help so that light does not start hitting your roots which is not a good thing. It will also help to keep some more water on the top roots.
Adieu
Adieu