Can't we find a common color for zoned buildings?
What about purple? Or another color? You can suggest any color that is distinguishable from the rest. I just think the greyscale isn't so good because grey isn't a real color, but a mixture of all colors, and doesn't fit into the system when there is yellow, red, blue, orange etc.
Also, the surfaces the people walk on use the greyscale, the roads are almost black, while the pavements are grey.