bug#74055: 31.0.50; color-lighten-name not lightening black

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Date: Mon, 28 Oct 2024 09:27:29 +0100
> 
> The function color-lighten-name doesn't work as I would expect for
> black:
> 
>   (color-lighten-name "#000000" 50)
>   => "#000000000000"
> 
> I think there is even a test that checks that, in color-tests.el:
> 
> (ert-deftest color-tests-lighten-name ()
>   (should (equal (color-lighten-name "black" 100) "#000000000000"))
> 
> I don't understand this, and it is different from what I'd expect, and
> tools for the web return, for example
> 
>   https://mdigi.tools/lighten-color/#000000

Our notion of "lighten color" seems to be to increase the color's
luminance by P percent.  Since the black color's luminance is zero,
increasing that by 50% still yields zero.

By contrast, the page you point to seems to interpret "lighten" to
mean that P is the percentage of the full scale, not of the original
color's luminance.

This goes back to commit 656c2dd66e, which was supposed to fix
bug#54514.  But maybe Noah's interpretation of "lighten" was
incorrect, and we should revert that change?  OTOH, if we do revert
it, then Noah's example will disagree with the above page.

So I guess our algorithm is incorrect somewhere?

Adding Julien and Mattias, in the hope that they could help unconfuse
us.