Apparently Japanese traditionally only had the four - what the article calls "red" was used for any warm color, and "ao" covered any cool color.

Interestingly, the usage was pretty fluid. Now that I read up about it, it seems that "ao" was used for anything from pale gray all the way to dark purple overlapping with black. This survives curiously in some quasi-archaic usage - 青毛 (lit. "blue fur"), meaning a black horse, and 青馬 ("blue horse") for a pale gray or white horse.