would have gone with a brighter shade of grey for a base of the city and left it with some serious underspray to make the light aspect look more real without too much more effort. then gone over the same stencil from a distance with the darker shade so the borders arent totally filled.
a friend of mine used to make some pretty amazing stencil stuff when he was in school, check it out
http://n0ddy13.deviantart.com/gallery/

look how he uses colour in his streetscape and the eifel tower
and how he uses the underspray in his pain/anguish/destruction pieces to create the light/glowing/blur effect