Although I do use branded pigments (mainly Vallejo) I wanted to try and make my own as a way of trying to get my own blends. The idea is very simple, take some chalk pastels and scrape the edge with a sharp blade and use the resulting dust. Although the dust isn't the finest it works well on bases, but with some extra grinding I see no reason why it wouldn't work as dust for vehicles etc.
This is the result after mixing it with some acrylic brush on varnish. I made them in to a wet paste and applied them with an old brush. I think they would work well for tred/tire mud or areas of heavy rusting.