Depending on how your room is shaped, you need to decide which walls are the night-time walls and which walls will become the day-light walls. I decided to split my room diagonally down the center and do half and half. I do caution against painting your entire room in the dark night-time themed colors, as I would think it would optically shrink your room and you would need lot of light in the evening hours to see comfortably. But it is your room--if you want a cozy, dark, cave-like feel to it, then go ahead and paint it all night-time!