Let materials create the contrast.
A restrained palette can feel rich when its materials differ. Linen beside smooth stone, woven fibres beside wood, or a soft rug against a clean floor adds depth without a loud colour.
Repeat with a little variation.
Bring one material into a few places in the room. The repetition connects the space, while different shapes and finishes keep it from feeling too uniform.
Choose for everyday life.
Think about how a surface feels, how you use it and how you will care for it. The most appealing material is one you enjoy living with.