Step 1: Overall Color

Airbrush an even coat of light orange all over. I mix a bright orange with white to town it down.

Step 2: Shading

Airbrush shadows with a slightly dark brown.

Step 3: Deep Shading

Airbrush the darker, deeper shadows with a brown.

Step 4: Overall Color

Airbrush an even coat of clear brown all over.

Example: