Ah, hehe, thats the point, not showing where it starts and ends!
Anyway, most of the pigments have been added to wheels and tracks, but also all lower parts of the vehicles, so as to add dust, dirt and grime. On the tracks I have also used rust coloured pigments.
Basically, you can aply the pigments in a few different ways. Either you start with a wet or at least moist surface and add the pigments, add the pigments to some liquid like white spirit or thinner or aply various pigments and then add liquid to mix and make the pigment stick to the surface. Naturally, one can also dust with the pigment, with a dry brush, and just add dust coloured, dirt or what ever one wants to achive, and aply it directly to where you want it, and this will yield a very dry looking result.
I hope this is of some help. My suggestion is that you go for MIG, AK Interactive or Valejo, buy a pot or two, and try out what you want to do. I have also gone to artists shops and got larger (and much cheaper) pigments of the basic ones, like ocra and such.