Great brickwork indeed !
As for the lamp post : yes, add rust if it's a steel one. However if you paint it copper/bronze you could use verdigris for a blue/green accent.
Also, since you don't have access to an airbrush there are some other ways :
liquid rubber/latex, dabbed onto the rusted model with a sponge.Just rub it off after you(ve painted your color over it.
Alternatively you could dab rust colors onto your already painted model with a sponge.
IIRC I used Maskol on these :

Oh, you'll need a sponge that's not too dense (or even nature sponge)
Also for rust : LifeColor's rust paint set is awesome, and for a faster method : modelmates rust effect (although this stuff is hell on brushes)
P.S. where's that building from or is it scrachtbuild?