Mostly it's a combination of tools. We write a lot of custom color functions (using Sass). You can check the src here[0]. A common one we'll use is makeHighContrastColor(), which given two colors will automatically make the first one a 4.5 AA accessible contrast by adding tint/shade.

For palette generation, we do a lot of our work with tools like chroma.js[1] which can help with some colorblind math.

[0]: https://github.com/elastic/eui/blob/master/src/global_stylin...

[1]: https://gka.github.io/palettes/#/9|s|00429d,96ffea,ffffe0|ff...