All of mine go though that stage there necks get lighter even the boys. You need to look at the wing and base of the neck. The boys will be barred feathers and the girl will start to lose the barred look and get the smooth brown look.
If you breed a Dark pied to a Pure IB. You can get IB with white pri. feathers, white neck patch. or a bird that looks like a Pure IB with no white.
Yeah Blackbird is right if they are Dark Pied. You can breed them to a white and get all Pied chicks.