I'm pretty certain that 3MF does have that support. I had ordered STLs for miniatures from Hero Forge before, which I converted to 3MF and was able to paint and texture with Microsoft's 3D Builder. So there is definitely colour and texture support. I'm absolutely certain that physical materials are also supported in 3MF based on examinations and experimentation with the format around the time I was digitally painting my minis.
The history of 3D formats is littered with weird stuff that doesn't really describe everything and needs separate texture files and metadata.
Meanwhile other things like text documents have embedded thumbnails and all kinds of stuff.
I'd expect 3D to have multi-material plus texture support(So the same file works in VR and 3DP) by now.