Here is some contractors wagon I have built I have tried to get the unpainted wood that silver grey colour that wood goes when weathered, Wood starts off a pale straw colour then goes sivler grey over time, many models seem to think unpainted wood is brown but if you look at the real world it is not.
Also the rusty steel work on the corner of the wagons is a shade of brown rather a red shade which seems what most modeller seem to think that rust is.