It’s always nice to mix things up a bit. I'm not real big on painting terrain. Throughout this thread, I think I only have two other pieces of terrain making this the third. It's an Imperial Bunker for Star Wars Legion I bought because it was half off at my FLGS. I almost certainly wouldn't have paid $65 for it, but $32.50 was just fine.
While you don't need an airbrush for a model this big, it's nice to have one. What I'm doing here is called under shading. I applied a black primer, then a patchy grey leaving the dark recesses alone, and followed that up by using a dark green for the shadowy areas. Paint is typically only semi-opaque, which means once I put the base grey color down those highlights and shadows should come through the final product. Once I get there, the next step is to begin the weathering process which will involve an oil wash, plants matter, pigments, and some glazing.

This is the exterior of the front door. I'm going for a weathered bunker, so the door looks weathered. I primed black, applied two coats of Army Painter Dark Rust and from there I sponged the following:
1. Vallejo Rust Texture
2. Pro Acryl Orange
3. Army Painter Uniform Grey with a few drops of Army Painter Speed Paint Ghillie Dew
3. Army Painter Uniform Grey
4. Army Painter Ash Grey
5. Army Painter Gun Metal
I followed that up by putting down some streaks using Army Painter Brownish Decay, but you can't really see it very easily in the photo.