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

In GNU Emacs 30.0.50 (build 1, aarch64-apple-darwin22.2.0, NS appkit-2299.30
Version 13.1 (Build 22C65)) of 2023-01-02 built on mini20.local
Windowing system distributor 'Apple', version 10.3.2299
System Description:  macOS 13.1

Configured using:
'configure --prefix=/opt/local --disable-silent-rules --without-dubs
--without-gconf --without-libotf --without-m17n-flt --with-libgmp
--with-gnutls --with-json --with-xml2 --with-modules --infodir
/opt/local/share/info/emacs --with-sqlite3 --with-webp --with-ns
--with-lcms2 --without-harfbuzz --without-imagemagick --without-xaw3d
--with-tree-sitter --with-rsvg --with-xwidgets
--with-native-compilation=aot 'CFLAGS=-pipe -Os -Wno-attributes
-isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk -arch arm64'
'CPPFLAGS=-I/opt/local/include
-isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk'
'LDFLAGS=-L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-rpath
/opt/local/lib/gcc12 -Wl,-no_pie
-Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk -arch
arm64''

Configured features:
ACL GIF GLIB GMP GNUTLS JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP NOTIFY
KQUEUE NS PDUMPER PNG RSVG SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP XIM XWIDGETS ZLIB

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8

Major mode: ELisp/d