This issue was already reported to KDE developers.
Read the comment from a developer please:
"Not sure if this can be addressed in QColorDialog. In Wayland, it is explicitely forbidden to read the screen contents. KWin offers an infrastructure for screenshot applications. I am not sure if the protocols used there use a standard that Qt could follow.
I suggest to report this to Qt developers (as a bug for QColorDialog)."
|For Gerrit Dashboard: QTBUG-81538|
|421215,14||add color picking support on wayland using the XDG desktop portal||dev||qt/qtbase||Status: NEW||+1||0|