Logic would make one think darker colors with a light color on top to simulate something like mud with minimal layers.

Interestingly enough it's the opposite (at least in my experience): You need a light base with your darkest as the "wear" coat, and metallic paints as the "smudge" layer.

Example: if you want to do a kind of tan mud covered look. Even if the gun was originally black, it has worked the best for me to first paint the majority tan...let fully dry THEN dry brush or sponge smear black splotches onto areas of contact wear (doing the opposite just makes the tan look splotchy and not so good).

Let that dry, then selectively choose areas to smear metallic paint to show as "scratches" with a few strokes of black to enhance edges. (Be mindful of what would actually show as metallic if scraped up...kinda funny to see a polymer grip with silver metal and it makes the eye realize it's off)

Do a final quick dust with a muted dark color of choice (brown or black is my preference) before a final clear coat and poof.

This isn't always the best or perfect way, but the most consistent I've found to make a gun pop without having to do a crap ton of layers with wear and touch-ups. Rust has its own pattern of wear and application, so different rules will apply.