Black.
That was the colour that was thought to best describe all things evil, including the devil. That was probably why everything in his office, or what most people called his lair, had that color; the walls, the desk, the curtains, his computer, his clothing and aura. The beautiful lights perched atop the room might as well have been the only thing that was not the being's favourite colour; they radiated a blinking purple light like the club below.
