I don't think the color zones are indicators of terrain type. (Though assuredly Whiterun is on tundra, etc.) Instead, the colored areas look like geopolitical zoning to me. We've been told that the guards in one area won't care about crimes you've committed in another area. I think we're looking at a jurisdiction map, among other things.
If true:
- It makes sense that the Throat of the World is not under the jurisdiction of any town.
- It would suggest that if you're wanted by the law, you can travel between zones by sticking to the mountains or coast, for the most part.