Mixing path & history
Your mixing history. The history is not saved, once you leave the page, it's gone.
Color mixing based on the selected paints' color notations measured on masstone. In some cases colors only have CIE L*a*b* data, on other cases also have the spectral reflectance curves.
When both colors have spectral curves you can choose mixing by spectral curves, and for some paints real painted mixing paths are available too.
Mixbox and Spectral.js are both using sRGB as input and output, and generate spectral curves from sRGB, do the mixing, and then convert back to sRGB. Read more about this method in the Practical Pigment Mixing for Digital Painting Mixpox paper.
All mixing methods are assuming the colors are opaque, and do not take into account the paint's individual properties like transparency, gloss, tinting strength, etc. How is it compare to real paint mixing? You can read a detailed test here.
For advanced paint mixing tools, check out: Zsolt M. Kovács-Vajna's Color Mixing Tools, Paintmaker (web version). You can play with Mixbox (custom rgb codes, n:m mixing) on Max Bo's Hello Mixbox Observable page.