Hi Jason,
I'm honestly not sure how common it is, as I've only experienced working with one breeding female NJ to date.
My girl has done this during every gestation. She is typically very clean wite on black, but somewhere between breeding and laying, her white becomes very dingy and dark. It remains this color until a couple of sheds after laying, when she seems to brighten back up over night with a shed. It's very interesting, I'd also be interested to know if of other northern type females do this (Delaware, Maryland, etc.) Thanks.
Will