Frank,
For OD and panzer grey vehicles I use a black undercoat and thin the color coat a little more than usual so that the recesses and inside corners come out a little darker.....spray angle will affect how this works for you. On the WWII German three color camo my base color is either MM Acryl rust or Polly Scale special red oxide to replicate the German primer. Again the dunkelgelb base coat is thinner than normal as are the camo colors.....hard to describe but it works similar to the above.
Apply decals or dry transfers now.
Next I drybrush with oil paint, on all but panzer grey I use unbleached titanium. On panzer grey I mix lamp black, panye's grey and titanium white to drybrush....several shades with a light shade in the center of panels.....then a final drybrush with unbleached titanium. Let the oils dry for a couple days.
Gloss coat with Polly Scale clear. For non-desert vehicles mix mars black, burnt and raw umber (pea sized blobs of each) and a very small dab of titanium white (maybe two pinhead's worth) and mix with Turpenoid (that little bit of paint in a 2oz jar filled with Turpenoid. Use this as a wash, I prefer pin washes except on cast surfaces. For desert vehicles skip the mars black add some yellow ocher to the umbers and some sienna (I use raw and burnt) and lots of white, you want a wash lighter brown than a Hershey bar! For openings like grills and vents on desert vehicles you can use a dark wash but for a general wash it's too stark. If the first wash isn't dark enough for you, do it again. Note-never use lampblack in a wash, it tends to clump.
Flat coat with Polly Scale clear flat, I thin it 40:60 or 50:50 with 45% isopropyl, spray at 40-50 PSI through my Paasche H from about 8". A light coat is all you need. Then comes fun with pastels. Go outside and look at your car then go look at a piece of construction equipment and some long haul trucks......what you want to notice is the dirt and dust and how they sit along with the color. For non-desert vehicles I grind up some browns, greyish browns, greys and a touch of ochre and a touch of pale pink and mix them in a jar. Take a soft, fluffy brush (if it feels good to tickle your chin, it's what you want) and liberally apply the pastel dust to the horizontal surfaces, just kinda dump it on with gentle stabs from the brush. Now get a pump bottle and fill it with distilled water......make rain.....mist the model from about a fott above and let the water start to pool on the horizontals and run down the verticals.....use a cheap 1/4" flat brush to helps steer the pastel dust where you want it under the water on horizontals and on verticals streak it down, vary the brush angle to vary the streak widths, remember dirt and water flow off projections.......as the water runs off and the pastels start to settle wipe the brush dry and use it to wick up excess water. In some places on horizontals you'll want a little more water because too much pastel settled there, just add a drop and use the brush to persuede the excess over the nearest edge and streak it down. On hard edges like fenders, let a little pool but not too much. Let dry.....you should now have one dirty vehicle. Seal with a light flat coat, if you want more dust you can rub it in with a stencil stump or cheap stiff brush. Rust can be done by rubbing a wet brush on the chalk stick then on the part. Dust for desert vehicles should be made from off whites, pale yellow ochers, pale umbers and pale pinks.....and there should be a buttload on the vehicle, especially in crevices.
If you want mud mix some acrylic or watercolor in gloss gel acrylic medium and some tea from a bag for texture...add a drop or two of retarder. Use a really cheap brush and apply where you want it.
Next is the coolest trick of all. Mix some water color and Polly Scale clear flat, thin it 50:50 as above, spray at high pressure and try for a
3/4-1" pattern at 12" distance. You're going to spray with a slight down angle on the vehicle sides, angle doesn't matter underneath. One thing to note, you want to see no color build for the first two mist coats. Using rapid passes start 2/3-3/4 the way up the vehicle and apply...do the whole side, front, read and bottom......come back to the starting level and repeat......as the color starts to show, drop the edge of your spray pattern when your start the next pass.....the goal is to simulate kicked up dust and dirt, get it heavier on the lower area and bottom.
The mud mix and the last trick can be any color you want. I vary the color mix by the terrain the vehicle would be in. I have a couple desert tanks (no mud) that the mix was an off yellow white, I've got some that are a blackish brown for opertions in rich farmland, some in very yellow ochres because that's the color of the dirt in the ops area and most in greyish tans as it's a good general dust color.
For oils, I like Winsor & Newton or the top of the Grumbacher line. Watercolors either of the above brands. Pastels, I use mostly Rembrandt loose sticks (I talked the manager at an art store into selling me the broken bits from the bins cheap). Le Carre makes a few boxed sets of patels anf the Basic Greys and Basic Earthtones sets are great starters. You want soft pastels, the chalk type, not oil pastels.