Prusa Open-Source Colormix for Easyprint and PrusaSlicer

The age-old problem: more colors without more extruders

We've been dealing with this for years. You set up your shiny new multi-material printer, load 4 filaments, and... that's it? Only 4 colors for everything? Sure, you can swap spools manually, but then what's the point of having an MMU in the first place? That's where the community got creative: what if we could blend those 4 colors to get 24, 39, or even more different shades?

The concept isn't new. 2D printing has used CMYK mixing to produce millions of colors for decades. But applying that to 3D printing is a different challenge — you can't blend molten plastic like ink. The solution came from a different angle: alternating ultra-thin layers of different colors that your eye perceives as a single blended tone. Think of it like pixels, but stacked vertically.

Ratdoux was the first to implement this with OrcaSlicer-FullSpectrum, and the community went wild experimenting. Justin H. Rahb built filament-mixer to predict the resulting colors. Projects like PeggyPalette helped share palettes. And Prusa, recognizing the potential, decided to build their own open-source implementation: the ColorMix model, available under the MIT license in the prusa3d/prusa-fdm-mixer repository.

What exactly is ColorMix and how does it work?

ColorMix is an open-source mathematical model that predicts the color you'll see when alternating layers of different filaments. It integrates directly into PrusaSlicer 2.9.6 and EasyPrint, letting you build palettes of dozens of colors using just 5 spools.

The key is layer interleaving. Imagine you're printing at a 0.1mm layer height. If you alternate one white layer, one black layer, white again, black again — from normal viewing distance, you'll see gray. It's the same principle as halftoning in 2D printing, just applied vertically.

Prusa's implementation uses the Yule-Nielsen model as its foundation — the industry standard for halftoning — but with corrections specific to FDM:

• Real prints come out darker than the pure mathematical prediction, especially when mixing very light and very dark colors
• Bright mixes lose saturation faster than dark ones
• Mixes in the cyan range (180–240° hue) tend to shift toward turquoise when they should stay cyan
• All of this is corrected with a weighting curve that leaves pure colors untouched

The model works in LAB color space for measurements and connects directly to the OpenPrintTag database for real filament data. It's available as an npm package (prusa-fdm-mixer) and has a single-header C++17 port for direct slicer integration.

The real limitations of the system (and why CMYKW)

Let's be honest: this isn't magic. The system has real limitations worth understanding before you get too excited. First, mixing ratios aren't infinite. With the standard setup you get 24 tones; drop to a very low layer height and you might reach 39, but not much beyond that. Why? Because practical ratios are discrete: 1:1, 1:3, and 3:1 for two colors, and 1:1:1 for three. If you wanted a 30:70 mix, you'd need 10-layer blocks, which eats into your vertical resolution and creates visible banding.

Second topic: the CMYKW set. Why five colors instead of the traditional three (CMY)? Because FDM is not inkjet printing. Alternating CMY layers never produces a true black — you get a muddy bluish-gray instead. That's why you need a dedicated black (K). And white (W) because there's no base "paper" like in 2D printing. So: Cyan, Magenta, Yellow, Black, White = CMYKW.

Third important point: the model is calibrated specifically for Prusament PLA on a Prusa XL. According to Ondrej Bartas (lead developer), the corrections should work similarly with other PLAs due to comparable pigment chemistry — but with PETG or ABS, the exact coefficients will likely differ. And that's before we even get to specialty filaments like galaxy or metallic, where angle-dependent reflectance completely breaks any scalar model.

What do you need to start mixing colors?

To use ColorMix you need a multi-material printer (MMU, AMS, or toolchanger), PrusaSlicer 2.9.6 or EasyPrint, and ideally 5 filaments in CMYKW colors — though you can experiment with any color combination you have on hand.

Minimum viable setup:

• A multi-material capable printer: Prusa XL 5T, CORE One INDX, any MK4S with MMU3, Bambu X1C with AMS...
• PrusaSlicer 2.9.6 (available on GitHub) or an EasyPrint account
• At least 2 filaments in different colors (obvious, but worth mentioning)

Ideal setup according to Prusa:

• 5 extruders or slots in your multi-material system
• A dedicated CMYKW set: Prusament is working on optimized semi-translucent colors, but in the meantime you can use Prusament Azure Blue (C), Ms. Pink (M), Pineapple Yellow (Y), Galaxy Black (K), and any white (W)
• Calibration models like the ColorMix Calibration Cones to test color combinations

Watch out for slicing times. EasyPrint has a 60-second slicing limit per plate. If your model is complex with lots of color changes, you may need to use PrusaSlicer locally. Not a dealbreaker, but good to know going in.

ColorMix vs other implementations: what does Prusa bring to the table?

Let's be clear — Prusa didn't invent layer-based color mixing. We all know that. Ratdoux with OrcaSlicer-FullSpectrum got there first, and Bambu Studio has its own implementation too. So what does ColorMix actually add?

The key difference comes down to calibration. While other models adapt paint theory or rely on RGB approximations, ColorMix was calibrated specifically by measuring real FDM prints with a colorimeter. Each test batch included the 5 base filaments plus 20+ blends, all measured in a single session to cancel out systematic equipment errors.

The result is more accurate predictions for FDM printing. Not perfect — Bartas himself admits they need more data — but much closer to what actually comes off your printer than models trained on oil paint.

Real-world applications: beyond the rainbow

Let's be clear: this isn't going to revolutionize functional printing. If you're printing gears, brackets, or mechanical parts, ColorMix means nothing to you. But for decorative, artistic, and miniature printing, it's a completely different story.

Where it really shines:

• Miniatures and figurines: layered gradients, armor, clothing with complex tones — no spool swapping needed
• Cosplay props: weapons with metallic gradients, armor pieces with smooth color transitions
• Educational toys: gradient puzzles that teach color theory hands-on
• Custom home décor: vases, lamps, and decorative pieces with unique color palettes
• Design prototypes: validate color schemes without buying 20 spools

And that's where our Anycubic Tough 2.0 resin at €22.99 comes in. ColorMix is great for FDM, but when you need fine detail on miniatures or ultra-smooth finishes on props, resin still can't be beaten. Think of ColorMix for large pieces with gradients and Tough 2.0 for the details that demand extreme precision.

What's next for the project: what to expect (and what not to)

Prusa has released the code under MIT and is actively asking the community for data. If you have a colorimeter and want to contribute, you can submit LAB measurements through their Gatherer app. The goal is to expand the model beyond Prusament PLA to cover PETG, ABS, and other brands.

What's coming, based on hints they've dropped:

• Prusament CMYKW set optimized with semi-translucent colors calibrated specifically for blending
• PLA Natural Glitter to add sparkle effects to any blend
• Model improvements for more efficient gradients
• Possible solution for top layer mixing

What you should NOT expect:

• True physical filament blending (it's still layer alternation)
• Pantone-level color accuracy (this is FDM, not offset printing)
• Consistent results with specialty filaments like wood-fill or metal-fill
• Continuous mix ratios (these will remain discrete by the nature of the system)

ColorMix FAQ

Can I use ColorMix with my standard Ender 3?

Not directly. ColorMix requires multi-material capability to automatically switch between filaments. You'll need some kind of MMU, AMS, or tool-changer system. A stock single-extruder Ender 3 can't take advantage of this technology.

Does ColorMix work with resin printing?

No, ColorMix is FDM-specific. Resin printing uses completely different technology — and resins can be physically mixed before printing, which is far more straightforward than alternating layers.

What layer height should I use for the best results?

The thinner the layer, the better the visual blend. 0.1mm gives great results. At 0.2mm, transitions become more noticeable. Going below 0.1mm helps slightly but not dramatically, and print times skyrocket.

Can I mix filaments from different brands?

Technically yes, but results may vary. The model assumes similar properties across the blended filaments. Mixing a matte PLA with a satin finish or a different manufacturer's filament can produce unpredictable color and layer adhesion results.

Why don't my colors match the preview?

Several possible reasons: your filament isn't in the OpenPrintTag database (so generic colors are used), the model calibration isn't dialed in for your specific combination, or the lighting conditions when viewing the finished part differ significantly. The model gives you an approximation, not an absolute guarantee.

Is it worth waiting for the Prusament CMYKW set?

If you already have primary color filaments, experiment with what you have. The optimized set will improve color accuracy and consistency, but it's not essential for getting started. That said, for professional work where color accuracy matters, it's probably worth waiting — or at least sticking to filaments from the same brand and type.

At the end of the day, ColorMix is one of those technologies you didn't know you needed until you try it. It won't change how you print functional parts, but it opens up a world of creative possibilities without having to buy 50 spools in different colors. And being open-source, it can only get better over time. The future of multicolor FDM is looking pretty bright — pun absolutely intended.

Happy printing 😎