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Idea was inspired by GNOME3 (when this DE was boring) about use different color scheme for each window.
Idea was born by possibility to mix accent color with brezee color + set to take accent color from current wallpaper.

It would be nice to differentiate, which window of some app window is. I can have ran many Firefox window - I known, each browser supports tabs currently, but... you probably understood. Another example - person could fast switch between gweenview window only looks at background around an image/other UI parts.

KWin will set different setup for each window of the same app with the same class (or other information supported by windowing system).

Solution is simple (select any mix of following):
1. Allow to set many color schemes and KWIN will set different for each window from the same set (app/window class)
2. Select random accent color for each window of the same set (app/window class)
3. Colorize window shadow with different color from the same set (app/window class)

Why? I do not use task manager (I set all my panels to autohide)
Also, borders in each window switcher could be colorized by the same accent color as selected by KWIN (if KWIN select accent color, so we do not use first idea), so: on window switch (alt+tab), on task manager (panel), etc.