Colors on websites should have good contrast with one another. This helps everyone understand the website.
There are a lot of good tools for when you need to check the contrast of two colors. Some even recommend colours.
Mine is the first that makes recommendations for 3 colors.
Sometimes websites have more than two colors. I am personally against this frankly insane design philosophy but I am trying to be less judgemental.
Examples of when you need three colors to match include:
- Buttons with a focus outline
- Some weirdo design like a menu with multiple colors
- Links without an underline (but don't do this)
The site uses the Chroma.js library for WCAG checking, and a lot of messy JS code in the front end for colour generation.
The main challenge is that often the needle for a 3 way compliance is so narrow, that floating point and rounding errors come into the results.