I thought the point was the vector-based rendering engine.
Given everything is shaded anyways, I don't see the issue with putting in a bit of colour, light and shadow.
Think about it, it would look just as good and the same style if say the grass was a subtle green, and the buildings were casting simple shadows that dimmed it. Same with the cars and the lights.
Nothing wrong with them being a single colour, but being black with the road being black. The only way to tell them apart is because they have lights. Yet you view Wireframe mode and clearly there is a box for the car and it's wheels which is being shaded black because it renders infront of the road.