I have no idea whether there was a standard color for brush guards as they were an accessory item. The grey color is likely a primer, allowing the guard to be painted whatever color the user wants.
As far as dealing with scratches on the bumper when mounting, what I'd do is paint the guard and the bumper the same color as the bumper AFTER it's been mounted to the bumper.
I doubt if there's any real way to prevent some chipping at the junction even after that's done. It would be best to get it secured to the bumper as securely as possible so there will be no movement, although if you tap something with the guard, a chip will surely occur at the joint.
