Why orange? I'm not entirely sure, perhaps because green, blue, gray, and white are already taken — reserved for representations of water, grass, housing, and roads respectively. The patches also seem to indicate a real-life geographic divide between commercial and poorer neighborhoods as seen in the image below.
In any case, all this went through my head as I captured the image above just outside the Farragut West stop on the orange line in DC. But unlike Google maps, there appeared to be nothing to see or do (at least at that moment) in this orange space.
The use of colors, as it's used in visualizations online and off, appear arbitrary but may contain subtle intention. After all, the map is not the territory; not to mention, orange paint was once more popular than the color blue.