Brown is not a hue, but a dark shaded version of Orange. Depending on the hue, you could try dark version of Yellowish Orange or Light Red as well. What this means that you can mix brownish color by shading Orange, or glazing Orange on top of Black.

Transparent Orange mixed with tiny amount of Titanium White. I only added tiny bit of black and white, so it's semi-transparent paint, showing the darkness of the Black Gesso, creating brownish outlook. Over-brushed Cadmium Red Medium here and there.

For the metal parts, base-coat with Van Dyke Brown, highlighted with Titanium White, rust effect with Light Oxide Red.

For the skull, Viridian coat with Titanium White highlight with a hint of Alizarin Crimson.

Reflected blue using a mix of Phthalo Blue and Titanium White. Working on the shield with Van Dyke Brown, Viridian, Phthalo Blue, Ivory Black, and Titanium White.

Base decoration. The bricks are made of epoxy putty using Greenstuff World's rolling pin, then glued sand, applied some washes.

Cured and varnished. Here are final photos.

Update: