A bedroom this pared back can take one serious dark blue. Open water fills the whole frame, so the wall stops being a blank and starts holding the room together. The dark lower half settles everything below it, while the light path draws the eye up and makes a low ceiling feel taller. Hang it above the headboard or on a narrow hallway wall where you pass it in low light. Keep the company simple: linen bedding, grey stoneware, pale wood or a dark stained side table, brushed steel fittings. Skip busy patterned wallpaper and warm terracotta tones nearby they fight the cool blue and flatten the shine on the water. As a canvas the surface stays matt, so lamplight never bounces off the dark areas.