SUMMARY If powerdevil isn't running, e. g. due to some error or because of > systemctl --user stop plasma-powerdevil.service the Brightness and Color applet, which normally lives in the tray, is currently fully hidden from the user. One can still add it manually (!) to the panel but outside of the tray to control the night light feature. It would be convenient if the applet did not mysteriously vanish, but showed an appropriate and understandable error, why its functions are limited in any case.