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3D printing tutorial

How to paint for 3D printing

Painting faces is how a single-color model becomes a multi-color print. In Forma3D you paint faces directly on the mesh, then use two automatic modes to tidy the paint job — merging tiny isolated islands or reducing the palette — before exporting a color-capable 3MF file.

Tutorial updated October 24, 2026 · Published by Forma3D · Screenshots show Forma3D on iPad. The same tools are available on iPhone; layouts may differ.

Close-up of a rounded orange cube in Forma3D with a yellow 2x2 patch painted on its front face
On this page
  1. Select the object and open Paint
  2. Choose a color and paint faces
  3. Paint to a target count
    1. Neighboring islands — Target 2
    2. Neighboring islands — Target 3
    3. Nearest RGB
  4. Tips: copy or merge colors manually
    1. Tip: copy a color
    2. Tip: merge colors manually

Note: Export and check in a slicer

Steps

  1. Select the object and open Paint

    Select the model in the scene, then switch to the Paint tool. Turn on the Wireframe overlay with the grid button if you want to see the boundaries of the faces you are coloring.

    Forma3D Paint toolbar with a magenta arrow pointing to the Wireframe grid button
    Tap the grid button to show or hide the mesh edges. The arrow marks the Wireframe toggle.

  2. Choose a color and paint faces

    Pick a color from the palette and tap faces on the model to paint them — here a 2×2 patch on the front face goes yellow. These colors become the per-face color data carried into a 3MF export. Use Single to color individual faces, or Brush, Circle, and Rectangle to cover larger areas. Fill colors a connected region and can stop at sharp edges, while Auto segments the mesh into regions and assigns colors automatically based on the selected angle.

    Before Rounded cube in Forma3D Paint mode, entirely orange, before any faces are painted
    After The same cube with a yellow 2x2 patch painted on the front face
    Painting faces directly: the cube starts fully orange; a few taps add a yellow patch.

  3. Paint to a target count

    Open the Target colors panel — the concentric-circles icon — pick a mode, set the Target with the slider, and press Paint to target. The Current counter shows the number of used paint colors in both modes. What Target counts depends on the mode: in Neighboring islands it counts connected painted regions (one orange region plus two separate yellow regions is 3 islands but only 2 colors); in Nearest RGB it counts unique colors.

    Neighboring islands — Target 2

    Merge small islands into their neighbors. Here the cube has a yellow 2×2 patch on the front and one isolated yellow face on the right — 3 islands in 2 colors. With Target 2, the isolated face is repainted to its largest neighbor.

    Before Cube with a yellow front patch and one isolated yellow face on the right side, Neighboring islands panel showing Target 2
    After The isolated face turned orange after Paint to target, leaving only the yellow front patch; Current 2, Target 2
    The isolated yellow face becomes part of the orange component — one yellow patch remains: 2 islands, 2 colors.

    Neighboring islands — Target 3

    The same mode removes scattered speckles. This cube has stray yellow spots on the top, right, and bottom besides the two main yellow areas. With Target 3, the tiny spots merge into their largest adjacent region while the front and top yellow patches survive.

    Before Cube with scattered yellow spots on top, right and bottom plus larger yellow areas, Neighboring islands panel showing Current 2, Target 3
    After The same cube after Paint to target with the isolated spots removed; the front and top yellow patches remain
    Small isolated yellow spots are merged away — the orange component plus two yellow regions remain: 3 islands, 2 colors.

    Nearest RGB

    Nearest RGB reduces the palette itself: it repeatedly finds the two most similar colors and repaints the second one to the first — globally, regardless of whether the regions touch. Here a 3-color cube (orange, yellow, and a green-turquoise patch) becomes 2 colors as the yellow areas turn orange; the green-turquoise patch stays.

    Before Orange cube with yellow front and top areas and a green-turquoise patch on the right, Nearest RGB panel showing Current 3, Target 2
    After The same cube reduced to two colors: yellow areas became orange while the green-turquoise patch stays; Current 2, Target 2
    The closest color pair is merged globally — three colors become two.

  4. Tips: copy or merge colors manually

    Tip: How to copy a color

    Open the Current color list. Drag the round color-picker control from the color you want to copy onto the round picker for the slot you want to recolor. All faces using that destination slot update together. You can also tap its picker to choose a new color.

    Before Current color list in Forma3D showing white, purple and green slots; the cube has purple painted faces and a green front patch
    After The same cube after dragging the white picker onto the purple slot: those faces turned white, the green patch is unchanged, and three slots remain in the list
    White is copied onto the purple slot, so its faces turn white while green stays unchanged. The three paint slots remain separate—even though two now look white.

    Tip: How to merge colors manually

    In the Current color list, drag the square swatch of the color you want to remove onto the color you want to keep. The source faces are reassigned to the destination slot across the model; the regions do not need to touch.

    Before Dragging the square green swatch onto the purple row in the Current color list; the cube has a white body with green front and top patches and purple speckles
    After The same cube after the manual merge: all former green faces are purple, the white body is unchanged, and only two used slots remain
    Green merges into purple. Only two used slots remain: white and purple. The green color remains available in the top palette for later painting.

Note: Export and check in a slicer. Export a color-capable format such as 3MF — STL does not preserve per-face colors. Then open the file in a compatible slicer and check the material assignments there before printing; the export step itself is not covered by the screenshots in this tutorial. Finish painting with Done before exporting.

Which mode should I use?

ModeTarget countsUse for
Neighboring islandsConnected painted regionsCleaning up speckles and tiny islands by merging them into their largest neighbor
Nearest RGBUsed colorsSimplifying the palette by merging the two most similar colors

Good to know

Does Nearest RGB need the regions to touch?

No. Nearest RGB merges the two most similar colors across the whole model, regardless of adjacency. Neighboring islands is the mode that works through shared edges between painted regions.

Will Neighboring islands keep a specific region?

It merges the smallest region into its largest neighbor until the target count is reached, so on a different mesh the surviving regions may differ. Disconnected regions with no shared mesh edges cannot be merged by this mode, so the requested target may not be reached. Review the result and use Undo if needed.

Do these modes change the model's shape?

No — both modes repaint faces only. The mesh geometry and its topology stay unchanged.

Is this the same as Auto Paint?

No. Auto Paint segments a mesh into regions and assigns palette colors automatically. The modes here work on the faces you already painted, cleaning up islands or reducing the color count to a target.

Related guides

Multi-color 3MF

Keep face colors through the export for multi-material slicer workflows.

Split STL by color

Use painted regions to split a model into parts with alignment joints.

Edit STL files

All the mesh-editing tools in Forma3D — selection, Boolean, cut, repair, and export.

Get Forma3D on the App Store

Frequently asked questions

How do I paint a model for multicolor printing?

In Forma3D, select faces or regions and assign palette colors, then export a colored 3MF. The per-face color data travels with the file into a compatible slicer.

Does painting work with any 3D printer?

Multicolor output requires a multi-filament setup — AMS, MMU, IDEX, or a tool changer. Single-filament printers will use the geometry but not the colors.

Which file format stores face colors?

3MF stores per-face color and material data. STL carries geometry only — exporting STL after painting loses the color assignments.