Flat paint is fine, but even gloss paints may appear flat with techniques used for weathering. You can use pastels, or other powders, for weathering. I usually use paint, drybrushed, or airbrushed with very fine, light spray. I also use Talcum powder, flesh colored (available at drug stores) for dust, on tires and lower parts of body. It is applied just like pastels and other powders, brushed on heavily and then excess brushed off until you get the amount you want. Repeat if it is not enough. Works on vinyl and rubber tires too, which paints do not work well on.
Some paint lines do have a dust color. If not, tan or "sand" color makes a fine dust color.