Weathering panzer grey

Weathering has nothing to do with the color of the tank,it is the environment you want to depict it in.If you really want to be accurate,you would need pigments to dust the vehicle especially the tracks and running gear.If you want mud then make that using the appropiate pigments,but if your not overly concerned,any dust,dirt,or mud pigment will do.Mig Productions makes a nice range.

You can also make a dust wash,or use a pre mixed one.

If you are making an oil wash,its the same thing,light dust,dark dust,Russian earth industrial dirt,sand,whatever environment you want your tank in.

Dry-brushing over the whole surface with a slightly-lightened tone of the base color is never a bad place to start. It simulates the 'scatter effect' of real light on a large surface -- basically breaking up the monotone blocks of color that your eyes never see as such in real life.

For weathering and wear, more lightened and more concentrated dry-brushing can then be applied to edges and raised detail. Then would come the mud and dust effects that Tojo spoke of.

Good luck.

This is roughly 5 different shades of gray tied together by weathering. I modulated the base colors by going dark on the lower part of the hull and places  where shadows would normally occur. It looked like a circus wagon until I used washes and pigments to dirty it up. 

Dry brushing, streaking, some dust effects and pigments. As mentioned, the colour doesn't matter on the methods you use.

Modulation is a purely artistic effect if you want to go that route, but i prefer to keep it realistic.

Unless you go really light on dust coats and weathering like mud, etc., the fuss over the base color goes right out the window...

If you're going for light weathering then modulation can still be something to try. I like to keep a balance of light modulation and weathering. But I've yet to complete a proper base for any of my models.

Modulation, if kept to a minimum can still help the overall finish and presentation. Works best with solid colors to break up an otherwise monotonous overall color tone.

With camo patterns it doesn't always work very well. So trust you're weathering to bring out the details.

 Examples;

Camouflage with no weathering (simple wash)

Vs.

It all depends on the look you're after.