Silver paint is extremely difficult to fix. Honestly (from experience), just leave it. If you touch it up with a brush, the metallic wont lay the same and you'll still see it, it'll just sparkle at you too. The only way to really fix it is going to be finding the appropriate color silver, sanding down the bottom of the car there, and spraying it with an airbrush. Even at that point, you're not likely to get a perfect match.
Are you certain its a scratch and not a scuff? You can try some ScratchX or other automotive polishing compound and see if it makes it a little less noticeable.
