Gun engraving is typically cut into steel or aluminum alloy surfaces. Both metals polish to a silvery surface that reflects light waves and, due to the preponderence of white light, appear silvery white (light) in color. The engraving naturally has the color of the base metal. So, engraving on polished metal is light on light. Typically, gun surfaces are given an oxide (of some kind) coating that darkens the surface (blue, black, case colors, annodized colors for aluminum). Now you have dark on dark. As the oxide coating wears off, you wind up with dark engraving on a lighter background. You can make the background light by polishing after hardening (or not coating if the part already hard) and fill the engraving with an "ink" coating to darken it = dark on light. In some cases, the engraving can be masked during coloring of the part or the engraving can be cut after coloring - light on dark.
French gray is one of many surface coloring treatments that dulls/matts a polished surface to a gray color. Uncolored is usually refered to as "coin" finished. Temperature colors (typically formed during the case hardening process, but can be formed or renewed without hardening) are usually noted as case colors. Single temperature oxide is usually blue or black.
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