What are the gray patches made from? Are they vinyl, leather, fabric (poly anything fabrics are notoriously hard to dye). If it is leather you can get a dye from any shoe repair or leather working shop (we had to do this with ballet shoes during Nutcracker season, people expect rats to wear black :-) ). If your shoe is vinyl you might experiment. Some paints will adhere some won't. It might be cheaper in the long run to just find a solid black pair. They're out there.