What color should I use as a wash for a model that has a German gray basecoat? Burnt umber? raw umber??

I use both Pnzr-Cmdr, but mostly i'd prefere raw umber.

HTH

Or an even darker shade of grey (than the base color) in variation with the raw umber.

Try black with about 15 - 20 % burnt umber. Makes a nice brownish black.
capnjock

Agreed... I mix black with Raw Umber for a great look... too much black looks like ... well - too much black! The real gritty color of an off-black is what you are looking for...

I've found the best color for dunkelgrau vehicles is Mars Black. It's an earth-tone pigment so the black has a definite brownish tinge. The others will look reddish or greenish on the gray.
So pick your color.

PH

Greets,

Now for the alternative, rebel view. Try 'negative washing' , e.g., like looking at the photo negative, light = dark, dark = light.

Washes are to 'enhance detail', 'pick out detail', etc.

Is the vehicle in dusty/muddy conditions? Why not 'wash' with the color of the environment? Vehicles tend to move which causes vibration; this tends to shift debris about, and gravity works! Dust and dirt will be rattled into low areas such as weld seams. It also 'seeks shelter' around raised objects such as rivets, bolts, handles and fasteners where the wind and feet and hands can not casually remove it.

I have a US in WWII in color book. It shows M4s in the California desert covered in dust. The glacis is pitted with fine imperfections, into which the dust has heavily settled. The dirt is also concentrated in corners and angles. Antwhere there is relief or depression, the sand has collected. It is 'negative washing' at its finest!

Find a hunter's vehicle, look at how the muck and dust n' dirt has blown and shaken and settled on it.

And be amazed!