Reason for the difference in map colors is probably because you can change the colors. This may also apply to cars, the speedometers. Like you could do in Need For Speed: Most Wanted.....
If not that then it could be an automatic thing. Where if the background is similar to the colors used in the HUD, it will change. So you can see the map more clearer, instead of turning around just to see if what you are looking at is correct. That would make sense but still not sure if that is plausible......Or when you enter new cities. Anyway there are a few ideas.
Anyway, these screenshots look cool. Very simple but effective. Now i can enjoy the city, instead of seeing how lovely my gun icon is in the top right!