8

you have a download in the old kde 3 style progress notification windows. by default window frame is blue. as the download gets close to its end the window frame gets darker. when the download is complete the frame is black

your computer is slow and some application uses the cpu for a while. the window frame of this application gets red (like it is heating). momentary splashes in cpu use won't affect the color but if the application seems to stay with high cpu load its window frame fades to red

a plasma notification appeared. the window that sent this notification becomes green for a while

if desktop effects are enabled this can be done as overlay or with some cool colors like the blue / orange bands seen in a soap bubble etc

if desktop effects are disabled this can be done thru temporarily changing the color scheme for this window (without fade effects)

Interesting... It will be a nightmare for theme designers, but I think the base idea is good.