The problem is Diamond Shield is porous, so the stains will probably be hard to remove.
The Diamond Shield website
recommends against abrasive cleaners and dry wash cleaners. They specifically mention Aerospace 303 Protectant and Protect-All, but those are more for protection than for cleaning.
Diamond Shield mentions a product called Novus for removing scuffs, so that may help for stains as may Plexus mentioned in the other post that Mike Cantor linked to.
Diamond Shield also specifically mentions Mother's and Meguiar's liquid cleaner wax, which also may be slightly abrasive to help with the stain.