Your grays are far too gray.. try adding a hint of color to them? Maybe a blue-ish or tan-ish color would help to pop the color in the tiles. And also.. your sidewalks are far too soft and lack contrast in comparison to the other tiles. Another thing.. try adding more depth to your sidewalks and making it look more like the sidewalk/street are connected, at the moment the sidewalks look almost as if they are just resting on top of the cement, when in reality sidewalks go beneath the leveled cement top. But other than those few things, it looks like you're off to a rather good start.
