I'm using plasma 5.12.5 with frameworks 5.43.0 and applications 17.12.3 on Gentoo.

I have a different desktop color set for each activity. However, only the color of the first activity that is open after login is used, for all activities. In the desktop background settings the right color is listed, but it is ignored. Also, if I for example select an image as a wallpaper for the desktop, that is ignored as well.

The only clue there may be is from the logs:

jun 01 08:11:20 <hostname> kwin_x11[11010]: QXcbConnection: XCB error: 3 (BadWindow), sequence: 61470, resource id: 35651742, major code: 15 (QueryTree), minor code: 0
jun 01 08:11:05 <hostname> kwin_x11[11010]: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 58731, resource id: 27287579, major code: 14 (GetGeometry), minor code: 0
jun 01 08:11:05 <hostname> kwin_x11[11010]: QXcbConnection: XCB error: 3 (BadWindow), sequence: 58730, resource id: 27287579, major code: 3 (GetWindowAttributes), minor code: 0

However, this may well be a coincidence that these messages appear when I want to change the background. They also appear at other times.

Before the upgrade to 5.12.5 setting the background mostly worked (it was fickle when attaching external monitors, but that is a different issue).