since you allready have diferent colors on eace face of the buildings, there really is no need for a black outline, or any sort of outline for that matter, as the diferent colors are enough for you to define diferente faces or areas or even objects.
my sugestion would be try to use what you have as an outline and transform that into an edge, that you can color acording to the direction of the light or too sugest a chafer.
maybe if you used a light color on the vertical outline you would see what I meen.
take care
Kaz