Barring almost if not always turns willow/green to yellow, black to yellow, and I believe but haven't personally seen enough, that it can lighten slate or make it more pink/white than slate. Not fully sure on that one though.
I think it depends a lot on the heritage/ breeding of the fowl.
I've seen Barred Rocks (and other Barred breeds) here in the U.S. and pictures of Barred Fowl from Germany that have little to no discoloring to the shanks.
Barred Rock (not my bird)