this is really simple. First of all I used dispersion-colors and painted everyting in grey. After drying I coloured the edges black, using an edding3000. After that I took just a little bit of black powder-pigments and gave them into water. This shoulld happen in a way, that the fluid gives just the idea of a soft black dust. Not more.
I wetted the call box all over with it. The result is a grey and dirty finish. Let dry. Then I used powder-pigments and dusted them (dry) with a very soft little brush onto the places, where I wanted to have earth, dirt, dry and wet rust, rests of green vegetation and so on. The rest of the pigments I brushed away with a hard brush.
You can get those powder-pigments in artist shops.
I hope I could explain my method in an understandable way. Explaining things like that I see very soon, that my English has narrow frontiers.
kind regards from the Odenwald