custom color picker

### This is a Feature suggestion
Using the default built-in color picker, or a hex text input, is great for accessibility (after all it's the topic of this repo) but it makes it hard to know which colors will tick the boxes. A custom color picker showing the area of the color space that will not fulfill the constraint could be a nice addition.

Look at the existing color input components in the codebase, likely in the button palette generation area. Research how to calculate and visualize WCAG contrast ratios in a color space. The feature needs to overlay a 'non-compliant' region on a color picker UI. Check for any existing contrast calculation utilities to integrate.

索引モデルが issue の本文から書いたものです。