Make sure you look at photos - there are two or three decent ones of very dusty Merkavas here:
http://www.twenot.nl/merkava.htm I would consider going over your dark / black wash with a sandy coloured wash instead, especially on the suspension, wheels, side skirts and then up onto the deck, and think about where it might collect. Lessen the dusty effect as you move up onto the turret.
You could probably make up washes with suitable coloured oil paints instead rather than having to buy specialist pigments. You can see there is not necessariily a "correct" colour - these photos show a range: pale creamy yellow, or more orangey, or even almost pink shades of yellow brown.
Another possibility is pastel chalks (not chalks, not oil pastels, but specifically pastel chalks from art suppliers.) These can be ground up with a file then worked into nooks and crannies, and are especially suitable for dusty effects, since essentially you are applying real dust. It is possible to blend colours together, although you can buy sets with a good colour range for under £10 (so probably only about $5 in the US!!) These will also help produce a more matt look.