See "Steps to Reproduce" Reproducible: Always Steps to Reproduce: 1. System Settings -> Display and Monitor -> Screen Locker -> Screen locker type: Desktop Widgets -> Configure... -> Settings -> Wallpaper -> Type -> Color -> First Color 2. When you click on the First Color selection button you get only semi-transparent window instead of color selection dialog.
Created attachment 76998 [details] screenshot
The screen locker architecture changed with Plasma 5. The classic screen savers are no longer supported. The 4.x series won't see any further feature development, so this bug report won't be implemented as it doesn't apply to our current version any more. I want to thank you for your bug report and for helping improving the quality of our software and I'm sorry that we were not able to provide a fix before we retired the affected component.