With the Light UI active, the current state of the K Only button is not visible, it remains in "inactive" mode:
This is especially annoying when wanting to apply K100 because there is barely any difference visible on the image itself.
Affects APu, ADe.
MacOS Catalina
MacBookPro9,1
Workaround:
Temporarily apply a color fill first to identify if the current K Only state is on or off.