set-foreground-color uses a color chooser: read-color.

customize-face uses something else which looks more readable IMO, though
the usefulness of the underline is debatable:

https://necromuralist.github.io/posts/changing-emacs-font-colors/color_selector.png

Other commands like set-face-foreground, highlight-regexp and others show only
color names, but not the colors themselves.

All color input commands should use a common color selector which shows the
actual colors too.