Pullman Car Roof Colour and Weathering

[proton]

I have an 8-car rake of OO scale Standard K and All Steel K-Pullmans, with three different roof colours.  The Standard Ks have the most realistic grey, although too shiny, so my intention is to (have my son) airbrush all eight with the closest grey, and then uniformly weather them with soot etc representative of the late 1950s.

 

We have an extensive selection of Tamiya paints - is there a recommendation for base "clean" colour, and weathering treatment please?

 

Thanks!

 

John