Wanted to make this ScaleTrains CP AC4400CW look like it had endured just a few months of service.
Blackened the grills below the radiator, and air intakes on the blower cab
Added a gray/brown filter to the wheels to give them a dirty (but not too dirty) look.
Airbrushed black around the exhaust stack and over top of loco from front to back -- then "washed" it off to allow black to settle around objects and into crevices. Then reapplied black around the stack with some spray over the top of the loco ... as if it had been through a wash rack a couple of times. Also did a light dusting of black into the radiator cores.
Airbrushed a brown on pilots/plow and around the entire area below the frame, with particular attention paid to the sideframes.
I then used the same brown onto the walkways and steps - varying the amount used to make it look like different parts of the walkway got used more or less.