Actually, it's enamel (powdered glass that is applied and fired to a glaze, then polished) and gold.

Some are quite extremely decorated, some even have jewels set in them.

This link is to an interesting Krieghoff catalog with everything from competition shotguns to some rather rococo - meets - moderne examples of the engraver's and jeweler's art on pages 53 - 62: Krieghoff

Enamel on silver.

Here's their "Hunting Collection" including doubles, drillings, single shots, repeaters... .
Krieghoff Hunting