bug#41544: 26.3; Possible incorrect results from color-distance

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Wed, 10 Jun 2020 16:51:24 +0200
> Cc: Tom Tromey <tom@tromey.com>, simenheg@runbox.com, 41544@debbugs.gnu.org
> 
>  (defun readable-foreground-color (color)
>    "Return a readable foreground color for background COLOR."

Please make sure the doc string says that the function will return
either the black or the white color, depending on which one will
contrast better with COLOR.  Otherwise it is impossible to know,
without looking at the code, that this function can return only these
two colors.

Other than that, I'm okay with the following parts of your patch:

  . the changes in list-colors-print
  . the addition of color-dark-p and the change in
    readable-foreground-color to use it
  . the replacement of css--contrasty-color with
    readable-foreground-color (assuming Tom doesn't object)

Please don't install anything else.

Thanks.