I spray black as a base coat, then mist spray Tamiya German Tank gray. Over this I spray Tamiya Neutral gray. Gives a nice 3D effect. Washes are done with shades of Burnt Umber artist oils, darkened with black for the crevasses, and lightened with a little cadmium yellow for the main areas.
Dust in the crevasses of the road wheels, etc is done by moistening the area with acrylic thinners, and then applying pastel chalks mixed with acrylic thinners. in these areas. When dry it looks just like ingrained dirt.
Over these I do a very light drybrushing with several shades of lightened Humbrol Tank Gray (#67?)
Finally a go over with pastel chalk dust completes the model.
Only thing I am finding is that the new Humbrol paints are not as good as the old formula for drybrushing. The pigment does not settle in the tinlet as well as previous paints. This can leave an unwanted shine on the drybrush run.
Panzer gray was effected more by dust than light. The layer of dust is more often than not mistaken as faded paint. Look at any dark coloured car that has not been washed for a while, you will see what I mean.