Even from top down, you'll want a light that comes from one direction, so one side of the roofs is more lighted than the other.
Then if you can implement cast shadows somehow, it would really make the constructions stand out.
Generally, background (flat) areas would deserve less saturated colors, so that anything above it (building, tree) pops more. It's something I noticed in 2D rpgs.
I checked from screenshots how the first SimCity did, and they chose oblique perspective for constructions, without going outside the limits of the 3x3 tiles (technically easier, they don't overlap the surrouding tiles)