The basic colors—Red, Orange, Yellow, Green, Blue, and Purple—are considered Common. Colors that are brighter or highly popular are ranked as High. Colors that are darker are ranked as Low. You must choose your color and live in this world. Treatment and recognition vary depending on your color. The living areas for each color are divided: those in the High rank live in the Sky City, those with Common colors live in the Surface Cities, and those in the Low rank live underground. It is possible to forcibly change one's color through illegal procedures, but the side effects are severe. Various illegal activities occur underground, while the Surface Cities are truly peaceful. You can travel to any city, but there are significant economic disparities due to color and status. The Sky City is safe, protected by both law and security; the Surface Cities have laws but no active protection. The Underground has neither law nor protection, and you must defend yourself.