Yeah, typically ICC profiles are used to make colour reproduction more accurate, but it should be possible to use one to make colours more vibrant/saturated instead of focussing on accuracy, the main hurdle will be creating a profile that does this how you want. Looking online it seems there are some tools that can edit/create ICC profiles like RawTherapee’s ICC profile creator and Argyll CMS, but these might be challenging to use to get the result you want.
For a simple solution you could try nVibrant if you have an NVidia GPU, or vibrant-cli if there is a Wayland compatible version (you mentioned it in your post, but from what I could find it only supports X11).
Gamescope can be run separately from Steam, but still has the issue that it will only work for whatever application is running within gamescope (unless you run the entire plasma desktop within it).
Someone has made a GLSL shader to increase vibrance, that is part of a kwin-effect-shaders project, but it hasn’t been updated in 3 years. If you are going to make your own KWin script/effect, then that shader might be a good reference.















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