First of all, tone down the bright white roofs. If they are supposed to be tin, then a silvery gray is the right color. Add some streaks of rust by dry-brushing (in the direction water would run off the roof. Tar paper can be simulated by using black construction paper; strips of 400 grit air-dry sandpaper (the black kind) can be made into convincing asphalt shingles. Dry brush some areas of lighter gray onto those types of materials.
Secondly, apply a bit of fading to the paint by using an airbrush and applying a very light mist of gray or white paint with an airbrush. Use a black wash to simulate soot stains over the engine house doors, and use the same to simulate grime running down from the corners of windows and the eaves of the roof. Using a drybrush, paint some patches on the wall a slightly lighter or darker shade. If one side of the building is shady and damp, some green to indicate algae growth on the walls is a good effect.
Overall, though, your best results will be obtained by finding a picture of how you want the building to look and work to duplicate that.
Connecticut Valley Railroad -- A Branch of the New York, New Haven, And Hartford
"We don't stop playing because we grow old. We grow old because we stop playing." --George Bernard Shaw