Something I learned the hard way: when you buy gray or silver shoes (or gold or bronze or your preferred metallic or any colorful color), it can be hard to find shoe polish that matches just right.
So here's my tip: when you first buy the shoes, before they need polishing, start polishing them with colorless shoe polish right away, and keep doing so.
If you didn't and now it's too late, Tarrago makes lots of colors of shoe dye/ polish, available on Amazon. You'll either get lucky and guess right as to which color you need or you'll end up with an expensive stash of polishes.