Currently 3mf files or loaded do not include color or material properties. This issue is about the official 3mf spec and not about multimaterial painting from slicers like Bambu/Prusa [wich was suggested in this issue]. I also made a comment there, wich is relevant for this Issue:
There also the official 3mf color and material spec. You can generate your files using this color implementation, with a Nightly build of OpenScad and other CAD software. I implemented my own loader for colored 3mf files, because trimesh did not support them.
This is my 3mf loader I use for 3DToLD
You can look at it for reference. I treat both color and materials the same and flatten the modeltree, keep these things may not apply to an official 3mf loader for trimesh. There are some placeholder sections for loading material painting for prusa/bambu, but I have not implemented them, since I could not find any documentation about them.
Here is also an example 3mf file that uses color(I think the color is applied per model and does not use triangle colors).
A model uses either colors or materials(I think you can not use both). They can by applied to a model and/or per triangle(triangles without an assigned color use the submodel color). Materials can have more properties than color for example transparency.
Currently 3mf files or loaded do not include color or material properties. This issue is about the official 3mf spec and not about multimaterial painting from slicers like Bambu/Prusa [wich was suggested in this issue]. I also made a comment there, wich is relevant for this Issue: