The one rule has got to be to stick within the right tone of grey. Some have a slight pink tinge, some might look blue toned, whilst others will look more silvery or even green based. As long as you stay within your spectrum of grey- albeit with more or less black and white added you will be fine.
The bedroom however is relying more on textures and simple patterns in the same shade to create a warm and interesting room. Remember using the same colour, but in different materials, will give you a pale contrasting shade.The subtle shaded wallpaper again seems to bring all the shades together more so than a flat painted wall would. And because there already seems to be enough subtle detail, a too busy paper would ruin the gentle balance of this room.
(images from housetohome)