It would be great if you could implement subtractive colors (like real pigment mixing > yellow + blue = green).
Digital "pigment" mixing is very rare to find on the web.
In fact I did a bit of research after I started working on this and it turns out Adobe did exactly what you're describing: https://research.adobe.com/publication/playful-palette-an-in... (the "learn more" button links to a detailed pdf with pictures of the interface)