I work at a smaller scale (6mm), so these techniques may be a bit simplistic for larger vehicles. But here is how I approach things.
I do both wash and dry-brushing.
For wash, I use acrylics.
All of my detail work, and often the base-coats, are done with brush and acrylic paints. But some models get a spray enamel base coat. In all cases I use a white spray primer first.
For my wash I use a highly diluted acrylic of near-black color. I have used "oily black" in the past when I could find Polly-S. Now I use a near-black dark brown called baby-poop (I kid you not), which my local hobby shop recommended.
I dilute about 10-to-1. I touch the brush to dishsoap before I use it to stir up the wash -- just the smallest bit of dishsoap works well to break the surface tension and help the wash flow. I completely blot the tank in wash. Then I do a second tank. Then a third. Then I panic and go back to the first tank and say "What have I done??" and use a toughed-up brush to blot away the wash. I press the brush in tissue to dry it, and blot again. Even twirl or push it around a bit. Then on to the second and third tanks.
I learned this technique of washing and "beating up" the model with a dry brush from Troy Ritter, who occasions these boards.
Then I dry brush with a color that is a light shade of the dominant color of the tank (usually base coat). Not usually the actual base coat with white (or yellow) added, as some do. Rather, I just chose a light color of the same basic tone (a very light green or tan).
Then I do detailing. Although sometimes some of the detailing is done first (like national insignia, which I may want weathered). After painting tracks (flat black for rubber tracks, metallic gray for metal tracks) I do a wash of the tracks and running gear with a rust color.
Here are some results.
GHQ SU-152s show how the technique performs on a base-coat (no-camo) model.
These GHQ M3 TDs were given a light camo pattern to simulate the mopped-on mud used by US forces as ad hoc camo in Tunisia. This, and the national insigna (yellow stars) were applied before the weathering.
These GHQ H39s (more properly H35 modifié 39) show some experimentation in various washes and dry-brushing. The bottom one (closest to the camera) was done with the technique I described above.
Hope that helps.
-Mark
(aka: Mk 1)