I don’t use it (as KDE has it’s own inbuilt Night Colour Control now), but in an effort to assist I did install it. Once you have redshift-gtk installed, you can find it via the menu > Utilities > Redshift
or launch it via konsole with
redshift-gtk &
FWIW, I note that it has a user systemd service that can be started automatically if desired…
Not sure if Leap 15.1’s KDE already has this, but TW’s has, in Systemsettings - DIsplay a “Nightcolor” option. Quite configurable AFAICS.
AFAIK the plasmoid needs activation from the systray’s settings ( it’s not active by default IIRC ). Right click the triangle to access these settings.