Why a Bambu Studio 3MF looks grey elsewhere

If a multi-colour 3MF from Bambu Studio looks right in the slicer but shows up grey in another viewer, the viewer is usually not broken. The file keeps its filament colours somewhere that a generic 3MF reader does not look.

Where the colours really are

The 3MF specification defines standard ways to store colour inside the model XML: base materials and a colour group. Many tools write colour there.

Bambu Studio does not put its per-object filament colour in those fields. Based on inspecting real exports, it uses two files of its own inside the same ZIP:

• Metadata/model_settings.config records, for each object, which extruder (a number starting at 1) it prints with.
• Metadata/project_settings.config holds a filament_colour list of hex colours, indexed by extruder.

A viewer that reads only the standard fields never opens those files, so it has no colour to show and falls back to one default shade. The geometry is fine. The information just lives in a place that is specific to Bambu.

When both exist, a standard colour written in the model wins, and the Bambu colours fill in objects that have none.

The empty-model version of the same problem

Bambu Studio also tends to keep the real triangles in separate parts under 3D/Objects/ and leave the root 3dmodel.model as a list of references. A reader that looks only at the root part finds zero triangles and reports an empty file. If a viewer shows “no mesh found” for a Bambu export, this is the usual cause.

Painted regions are a third case

Both Bambu Studio and PrusaSlicer let you paint colour onto parts of a surface with a brush. That data is stored per triangle in a compact, slicer-specific encoding, not as normal colour. A viewer can read the base colours and still not draw the painted areas.

MeshView detects these files and shows a one-time notice that painted regions are not drawn, so a flat result is not mistaken for the real colours. It does not decode the paint data.

What to check when colours go missing

1. Open the file in the slicer that made it. If the colours are there, the file is fine.
2. Look at the extruder and filament assignment in the slicer. A model assigned to a single filament has one colour on purpose.
3. Try a viewer that reads the Bambu files. MeshView reads both sidecar files, so a plate with several filaments shows its filament colours.
4. Do not convert to STL to share colours. STL has no colour field, so everything would be lost. See STL vs 3MF.

What this means when you share files

If you send a Bambu 3MF to someone, their viewer may show grey even though the file is correct. Tell them the colours are Bambu-specific, or send a screenshot from the slicer next to the file so they know what to expect.