Practice on some clear acetate first -
Take some Tamiya Smoke (can't remember whicn number) and add a little India ink (just a little at a time) and airbrush on several layers of this mix.
You should be able to gradually darken the windows, layer by layer.
Remember, in real life it's illegal to tint the windscreen and rear windows of a passenger car (well, it is here in OZ). The sides can be tinted, but not the front or back.
Cheers
LeeTree