White and cream are both neutral colours, meaning that they go with any other colour, including each other. The challenge when working with cream walls and a white comforter is deciding what other colours you want to integrate into the room based on the mood you want to create.
Psychology of White
In the Western world, white is associated with purity, safety and creativity. It not only feels safe but is also a safe bet when decorating. White can be paired with any other colour and end up looking equally stunning.
A monochromatic colour scheme is achieved by using varying shades of the same colour. You're halfway home by combining cream walls with a white comforter. Expand upon that theme by layering more of the same into the space. Paint the ceiling a pure crisp white or a slightly lighter shade of cream than the walls. Introduce off-white furniture and a snow white area rug and curtains. Paint the room trim the same intense white as the comforter, and accent the bed with cream-coloured pillows and throws.
Choosing a Mood
One of the advantages of cream walls and a white comforter is that you can set any mood you'd like. Introduce vitality and excitement through the use of "warm" coloured accents. If you want to remember which colours are warm, think of the sun. Red, orange and yellow are all warm, exciting shades. If you'd rather, you can opt for a calm, tranquil room by introducing "cool" coloured accents. Cool colours are those you might see in a snowy mountain vista. Blue, green and violet are all cool, calming shades. Introduce either mood through accents like lampshades, area rugs and artwork.
You've already set the stage for simplicity by choosing cream coloured walls and a white comforter. Carry that clean, simple decor out by limiting the number of things you bring into the room, leaving plenty of open floor space and organising so that everything has its place.