Looking at the github, the code license is proprietary for commercial use, as this would be.
A free comparable solution might be using OkLab (with easy additions of OkHSL and OkHSV for color pickers too). That's a perceptually linear color space so functionally the same as what Mixbox is solving (mixing colors in a realistic way), though the math is a fair bit simpler since it's dealing with relatively simple colorspace translations instead of simulations of paint pigments. There are freely licensed code implementations of the algorithms in a fair few languages, too. It's gaining widestream support rather rapidly (Adobe themselves added it to Photoshop, yet not Illustrator yet), so it could be another marketable feature (tucked into the "Color Control" section on the homepage maybe).
There's already a feature request for adding OkLab, looks like:
It would be great if Affinity products started supporting perceptual color spaces for image processing, like Björn Ottosson's Oklab.
Let me quote two sections from Björn's post:
References:
https://bottosson.github.io/posts/oklab/ https://news.ycombinator.com/item?id=25525726
https://raphlinus.github.io/color/2021/01/18/oklab-critique.html https://news.ycombinator.com/item?id=25830327
mws got a reaction from moi.cool in Mixbox colour mixing is very realistic