On Mon, 30 Jan 2023 21:48:20 +0000 "Mark Bestley" <gnu@bestley.co.uk> wrote:

> Look at the results of
>
> (require 'color)
> (message "reduce by 100 = %s" (color-lighten-name "Black" 100))
> (message "reduce by 0 = %s" (color-lighten-name "Black" 0))
>
> In emacs 28.2 they give "#ffffffffffff" and 0 as expected.
> In emacs 30.0.50 they give 0 and 0

This difference is due to this commit:

commit 656c2dd66e77a5fbeb99d358017e8327401fae05
Author:     Lars Ingebrigtsen <larsi@gnus.org>
Commit:     Lars Ingebrigtsen <larsi@gnus.org>
CommitDate: Tue Mar 22 15:28:02 2022 +0100

    Fix color-lighten-hsl logic

    * lisp/color.el (color-lighten-hsl): Lighten by percentage,
    instead of just adding the specified number to the luminance
    element (bug#54514).

The patch below restores the Emacs 28 result for the above examples
while keeping the desired result for the example in bug#54514, but I
have no idea if it yields undesirable results in other cases.

Steve Berman