How I paint armored vehicles (...without an airbrush):
1. Prime either black or gray. I use gray if the vehicle will then be a green/khaki color so it's not so dark.
2. Spray with desired base color.
3. I am testing two methods, but I apply a black or dark grey wash over the rivet areas and joints. I was using Vallejo Black Wash (darkened) but have switched to thinned Reaper Grey Liner. (The other method is to apply the wash after the body is completely painted.)
4. Using a stipple/deerfoot brush, lightly apply a comparable or slightly lighter color in the main areas and close to the rivets to blend the darker washed area into the base color.
5. Apply more color paint to the centers of the open areas to lighten them a bit more. Or, do the same with a lighter color. This adds contrast between the open areas and the rivet/joint areas.
6. When you're happy with the paint job, if necessary go back and line the joints, slits, etc. with the black/grey wash.
7. Paint other details -- guns, lanterns, tires, etc.
8. For additional contrast, drybrush the rivets with a lighter color. Optionally, also highlight the edges.
9. For weathering, again use the stipple/deerfoot brush to dapple browns for mud.
Sounds lengthy and involved, but it's really quite quick and simple.
Yes, an airbrush would be better. But for those of us without one, this method works well.