Hallo wolfn,
Super! Your approach also brings success.

Now I know why this change didn't work at first: Because the following was in the .profile: export QT_QPA_PLATFORMTHEME=gtk2
If I replace there gtk2 by qt5ct it works again.
Or delete the line in ~.porfile and write the correct one in /etc/enviroment

These are the settings of a clean and completely reinstalled system on Manjaro Linux:

~.profile
export EDITOR=/usr/bin/nano
export QT_QPA_PLATFORMTHEME="qt5ct"
export QT_AUTO_SCREEN_SCALE_FACTOR=0
export GTK2_RC_FILES="$HOME/.gtkrc-2.0"

and

etc/enviroment
#
# This file is parsed by pam_env module
#
# Syntax: simple "KEY=VAL" pairs on separate lines
#

Thus, the Color Picker appears exactly as you would expect.