Weather or shade/highlight?
What I'd do is paint the base color. I'd then take that green and add red and lay that into the shadows. I'd take the base color again and add a yellow or beige to it and paint on the highlights. I'dincrease the value of both the highlight and shade by adding more of the beige/yellow and maybe a bit of very dark brown to the red green mixxture for the deepest shadows and highest lights. It's best, in general, to avoid white or black as tinting agents. I would not do a wash except for very textured surfaces. Same with most drybrushing, though I just used a bit of that technique to blend the edges of a highlight and base on a dark blue shirt today.
To show dirt, after doing what I described above, I'd lightly brush on earth colors or use pastel chalks. Remember, the darker the base, the less general dirt will show. Think of your jeans. When they're fresh from the store, assuming a pair that looks new, most dirt won't show up unless it's really dark stuff like oil or blood. As they get faded, dust and other dirt will show more contrast with the pallette provided by the lighter color.