I use pigments from MIG and Tamiyas weathering sets for coloring. I have first mixed natural rust and panzer gray pigments with 96% alcohol to make a colorsoup that wont wrinkle the paper in the same way it had done if I mixed with water.It dries very fast and the colors sets pretty good.On top of the rusty color I have put some gunmetal/polished steel color from thw weathering sets also mixed with alcohol. I have some access to rectified alcohol so I just grab the nearest thing I have also thinned the pigments with white spirit with the same result. On the edges and here and there on the tracks I put some dark mud pigments. And on top of it all when finished I will brush with "desert sand" pigment. I dont seal anything with matte spray....I just leave it as it is. I think that if spray is applied to the tracks they might become very stiff and not easy to move when the varnish have sealed the tiny gaps between the links.