I ended up giving it two sludge washes, one brown/black and one dark green.
With a layer of green spatters, a brown-black pin wash and a second layer of light brown spatters in between
I finished it off with a wash of the Vallejo ready made "desert sand wash". First time I used it, and it worked quite well.
When applying the washes and the dirt I tried to approximate a more natural distribution, so the lower half of the Kuster and lower 2 thirds of the Friedrich have more dirt then the top parts. Trying to keep the dirt line at the same hight off the ground on both...
Then I put on the final matt clear coat after which I added some gloss varnish details (to mimic oils spills) and put some graphite on the the bare metal parts and the engine...
Here's an overkill of pictures.
Hope you like it, feel free to leave a comment either way.