Painting Colors on a Model in the Slicer Before a Multicolor Print

Painting the surface vs. swapping filament at a layer

There are two ways to get more than one color on an FDM part. The first is a layer-based color change: the printer stops at a chosen height so you can swap spools. According to the Prusa Knowledge Base, PrusaSlicer writes an M600 command into the G-code, the printer pauses and prompts you to continue. You drag the layer slider in the preview, click the plus icon, and repeat for as many changes as you need. This is all a stacked logo or a nameplate needs, and it works on any single-extruder machine.

Painting in the slicer is a different idea. You color the surface of the model directly with brushes, and the slicer assigns each painted region to an extruder or a slot on your filament changer. Colors can sit side by side on the same layer, on the walls as well as on the top. That requires a machine with a multi-material unit or several extruders, which is where our guide on choosing a multicolor printer comes in. Prusa's documentation notes that preparing models this way used to be slow and complicated, and that since PrusaSlicer 2.4 it takes just a handful of bucket fills and brushstrokes.

The brush, smart fill, bucket fill and height range tools

Per the Prusa documentation, the multi-material painting tool works like the support and seam painting tools: hold the left mouse button to paint with the primary color and the right button to paint with the secondary color. Three brush shapes are available. The sphere and circle brushes have an adjustable size, plus an option to split large facets into smaller ones while you paint, which gives you a cleaner outline on a low-polygon model. The triangle brush uses the model's own triangles, so it is the right pick when you want to follow an edge the designer already built in.

For areas bounded by crisp edges, smart fill saves the most time. Hover over the model and you get a live preview of the region that will be painted: it spreads out from under the cursor until it hits a sharp edge, with an adjustable threshold angle. The bucket fill swaps one patch of color for another, again with a selectable angle. The height range tool paints everything between two horizontal lines whose spacing you set, which is handy for a clean color band around a vase, a box or a display base.

Mapping extruders to your mouse buttons and checking the preview

In the tool panel, two combo boxes labeled first color and second color map one extruder to the left mouse button and another to the right. The number and color of the selected extruder are shown so you always know what you are laying down. If the default swatch does not look like your real filament, click the colored square next to the extruder and pick something closer. Prusa's documentation is clear that this visualization color is not saved anywhere and is lost when the application closes; it exists only to help you tell regions apart while painting.

Once the regions are painted, slice and switch to the preview. This is where you check the result layer by layer: a forgotten spot on the back of the part or a brush slip across an edge shows up right away. Make a habit of rotating the model through every angle before you commit. In PrusaSlicer, painting happens in the prepare view, and the sliced preview confirms which extruder actually owns each region. The painting never alters the mesh itself, and the colored regions cannot be exported as separate bodies.

How brushstrokes become printable regions inside the part

Painting a surface is not enough to print a color; it needs thickness. According to the Prusa Knowledge Base, the slicer segments the surface into patches of the same material and then extends each patch inward. On top and bottom surfaces, the extension matches the number of top solid layers and bottom solid layers set under layers and perimeters in the print settings. On side walls, the region is expanded inward up to a maximum width controlled by a parameter called mmu_segmented_region_max_width, found under the advanced multiple extruders settings.

That inward extension has a specific purpose: it gives the different colors or polymers the best chance of bonding together, through an overlap in depth rather than a thin boundary at the surface. In practice, a pattern painted on the top of a plate prints through the full thickness of the top solid layers, and a pattern on a wall occupies several perimeters. If you want a color that runs through the whole part, raise the solid layer count or the segmentation width, keeping in mind that more color per layer also means more filament changes and more purge waste.

Thin walls, extruder count and XY compensation: the limits

Prusa's documentation reminds you that thin features are not printable with FDM, and painting does not change that: painting a thin wall in two different colors will not make it printable. Thin, unprintable regions that appear during segmentation are detected and merged with the neighboring region, which changes their color. If your design has fine lines or lettering, turn on detect thin walls, which Prusa recommends trying in exactly this situation, then check the preview to see what survived slicing.

Two more limits are documented. Multi-material painting supports up to sixteen extruders; if your printer profile declares more, only the first sixteen appear in the painting tool. It also cannot be combined with XY size compensation, which is ignored during slicing as soon as an object is painted, and a notification tells you so. If you were relying on that compensation to dial in a press fit, adjust the dimensions in CAD before painting instead. The same brush logic also drives painted supports, so the hand skills you build here carry over.

Getting ready to print: matching materials, purge waste and saving the project

The Prusa documentation stresses one rule that applies to every multicolor method: use filaments of the same type, PLA with PLA and PETG with PETG. Mixing different materials will most likely cause a failed print because they do not adhere well to each other. Pick your colors from one material family, ideally from the same manufacturer so the temperatures line up. Every color change on a layer triggers a purge, so before you paint a busy pattern, estimate the waste with our multicolor purge simulator.

Finally, save your work as a 3MF project rather than an STL. The painting does not touch the mesh; it lives in the slicer project, and an exported STL would come back with no color at all. A 3MF also keeps the extruder assignments and the plate layout, so you can reopen or share a painted model later. If you do not own a multicolor machine yet, our buying guide and the startup budget tool will help you place entry-level and mid-range options without committing to a number.

Sources and limits

• Prusa Knowledge Base — Multi-material painting
• Prusa Knowledge Base — Color change

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.