Its actually easier than you think.
Spray a appropriate base coat over the debris, say a medium gray. Brush paint bits a different color like brick red, some darker, some lighter. Include different colors and shades of gray. Use a wash to darken the shadow areas, I try and avoid black. If coated in Future or acrylic paint oil paint thinned works well, Burnt Sienna is a preferred color. Then hit the tops of the debris with a light dry brush of light gray. After everything is in place the last step is to tie it all together with a pigment powder, a light brown or light gray to simulate dust. Get it on the figure(s) boots, pants and anywhere else they may have gotten dirty! Combat is messy business. Dust the vehicles heavy on the lower portions and light on top. A cannon firing in a debris field raises a lot of dust. Now this is done do not clear coat or touch! Both will mar the effect.
The main thing is stick to your theme (location) If buildings are sand color so should your dust be, gray works well for European cities, red-brown for locations like Vietnam. Uniforms like young kids attract dirt quickly, under fire soldiers take cover ASAP without regard to how dirty they become. Soon they resemble the color of the theater of operations they are situated in.