Nothing in the military stays clean, unless you scrub it! Vehicles are generally dusty, or muddy, depending on the weather. Drybrushing can attempt to simulate this, but the best tool is an airbrush. Apply a mist of light tan, or light grey for dust, reddish brown for mud. Go lightly here, too much is bad. Dust can also be used to lighten models that got a bit too dark.

Here is a 1940 French Char B1, with a base of deep yellow, and a hand application of earth brown and hunter green.
The camouflage colors are outlined in black. A very fine tipped black felt tipped pen was used. The mufflers have been "rusted."
An airbrush has dusted the scheme, and an aerial has been added: .015" bit in a pin vise to drill a hole, and nylon thread glued in. Color aerial with black india ink.
A winter KV-1 1940 in white base, grey wash, and then white drybrush. Road wheels are grey, with tracks and cables rust & silver.
Weathering is applied by hand, with washes and spattering of some rust brown paint. Acrylics are great here: if too much, simply clean off with a watery brush.
Paint up one complete unit at one time. This insures that the colors are uniform. This can ease tabletop identification, some units being "cleaner" and newer than the more veteran fighters.