Tutorials · Published on 15 October 2026 · By the usi3d team

How to paint a 3D model for multicolour printing and export it as 3MF

To print a model in several colours, every face needs a filament assigned to it and that assignment has to be saved in a 3MF, because STL stores no colour. At usi3d you do it in the browser for free: open the STL, GLB or 3MF in Colours and filament, paint by area or with a brush, reduce it to the filaments you own and download a 3MF that Bambu Studio, OrcaSlicer and PrusaSlicer can read. Since 3 November 2026, the download comes with the Pro, Max or Business plan.

What you need to print in several colours

In FDM printing there are three ways to get a part in several colours, and each needs the model prepared differently. The cheapest works on any printer; the other two need hardware that swaps filament by itself.

If your model changes colour by height (a sign with raised letters, a planter with a stripe), a pause at the right layer in the slicer and a manual spool swap will do. A figurine with eyes, beak and wings in different colours needs specific faces painted, and only a multi-material system or a multi-toolhead machine can print that.

Why an STL does not keep colours

An STL is a list of triangles and nothing else: no colour, no material, no units. Export a painted figurine to STL and it reaches the slicer in a single colour. For the slicer to know which filament goes on each face you need a 3MF, which stores the geometry in millimetres together with the filament assignment.

AI-made models usually come as a GLB with a texture: an image with thousands of shades, shadows included, wrapped around the mesh. A slicer cannot turn that image into filaments. It first has to be grouped into a few solid colours, one per spool, and cleaned of the specks left by the photo's shadows. The differences between formats are covered in STL, OBJ, GLB and 3MF.

How to paint a 3D model in usi3d, step by step

1. Open the model in Colours and filament

   Go to Colours and filament and open your STL, GLB or 3MF, or try one of the samples, each with its licence. Everything runs in your browser and uses no credits.

2. Choose how to fill

   With Fill, one click paints in one go. Area paints the region of the colour you touch and stops where the colour changes, at a sharp edge or where it meets another piece. All that colour changes it across the whole model. Surface spreads until the surface bends more than the angle you set, and Whole part paints the part you touch, whatever its colour.

3. Refine with the brush

   The Brush paints as you drag, sized in millimetres; the [ and ] keys change it. Use it for details the fill cannot separate cleanly, such as the rim of an eye. The Picker takes the colour of an area so you can keep painting with it.

4. Copy one side onto the other

   On symmetrical figures, paint one half and use Copy one side onto the other (left and right, front and back, or top and bottom). It only copies where both sides match: on a posed figure, not everything has a pair.

5. Reduce to your filaments

   Under Filaments to print, choose from 1 to 8 filaments and press Reduce, no specks. The tool groups the colours, removes specks from shadows and stray pixels and leaves clean edges between colours. If the part has fewer distinct colours than you asked for, it tells you.

6. Compare with the original

   Switch on the original view to compare against its texture. If something is not right, you have Undo and Clear the paint.

7. Download the colour 3MF

   The 3MF carries the filament painting that Bambu Studio, OrcaSlicer and PrusaSlicer read, with the edges between colours subdivided so they print cleanly. You can also download a GLB with the same colours, without the original texture. Both downloads come with the Pro, Max or Business plan.

How to take the 3MF into your slicer

1. Open the 3MF in Bambu Studio, OrcaSlicer or PrusaSlicer. If the program asks whether to load it as a project or geometry only, choose geometry and keep your own printer profile.
2. Check the filament list: each painted colour takes a slot. Assign each one the real spool you have loaded in the AMS, the MMU3 or whichever feeder you use.
3. Review the prime tower and the flushing volumes. Going from black to white needs far more purge than white to black, and slicers work it out per colour pair.
4. Turn on flushing into the object's infill or supports if your slicer offers it: whatever is purged gets used inside the part.
5. Slice and step through the preview by colour, layer by layer, before sending the job.

How much filament goes on purging

Every time the printer swaps filament it has to push out the old colour until the new one runs clean. usi3d's Save filament page works on about 435 mm³ per change on a Bambu Lab printer with an AMS, just over half a gram of PLA, and about 140 mm³ on other machines.

Run the numbers on a 60 mm figurine at 0.2 mm layers: that is 300 layers. If every layer has two colours, the slicer needs at least one change per layer, roughly 300 changes. At half a gram each, that is about 160 g of purge for a figurine that might weigh 40 g. So it pays to keep small colour details within a few heights, flush into the infill and skip painting a 2 mm detail nobody will see. Bambu Lab explains the options in its guide to reducing waste during filament changes.

Choosing the spools for each colour

In Colours and filament, the Which filament to buy panel takes your part's palette and, for the material you pick, finds real spools of a similar colour in the shop, with their availability. The match is approximate: light, screen and the filament's finish all shift the shade, and a matte PLA and a silk PLA of the same colour look different on the part.

Common mistakes when printing in several colours

• One colour bleeds into the next. The purge was too short for that colour pair; raise it in the slicer. The causes are in the guide to purging and colour mixing.
• The AMS will not load a spool. It is usually the filament tip, a tangled spool or a reel outside the supported size: see AMS not loading. On Prusa machines, MMU3 not loading or jamming.
• Strings between colour areas. Lots of changes and travel moves mean more strings; check retraction and filament moisture in the stringing guide.
• The part has holes or odd faces. If the mesh has defects, painting and slicing fail in those areas. Repair it first in Repair an STL model, as we explain in how to repair an STL.

Try it on usi3d: Colors and filament