I am unable to assign ICC profile in KDE’s settings > color management section - assign button is grayed out.
I tried KDE on two different computers and on both I could not set ICC profile (both have NVIDIA).
Also when screen goes to sleep after wake up its brightness changes.
Initial support for applying icc profiles will land in Plasma 6,
Check this: These past 2 weeks in KDE: Wayland color management, the desktop cube returns, and optional shadows in Spectacle – Adventures in Linux and KDE
This only applies to Wayland. OP did not specify whether they are using X11 or Wayland