When color and material are kept simple, texture is what keeps a room from feeling flat.

A minimal palette removes a lot of the usual tools for making a room feel layered — you're not leaning on contrasting colors or a wide material mix. Texture ends up carrying that weight instead: a rough plaster next to a smooth stone, a matte finish against a soft fabric, a woven surface catching light differently than the wall beside it.
None of this is loud. It's the kind of detail that registers as a feeling before it registers as a decision — a room feels rich even though, described on paper, it's fairly restrained.
It's also one of the harder things to plan for on a drawing, which is why so much of it gets resolved with samples in hand, in the actual light of the actual room.