SUMMARY
I tried to select my accent color with by using System Settings > Appearance > Colors > Custom > Pick Screen Color. The Color Picker from that dialog won't pick screen colors. I opened System Settings from the terminal, and tried Color Picker again.

I saw this message : "This plugin supports grabbing the mouse only for popup windows"
Following that, this message was repeated numerous times: "QImage::pixel: coordinate (0,0) out of range"

STEPS TO REPRODUCE
1. Open System Settings and navigate to Appearance > Colors
2. Click "Custom" under "Use accent color"
3. Click "Pick Screen Color" and attempt to pick a color from your screen

OBSERVED RESULT
The cursor turns into a crosshair shape while within the popup window, but does nothing when clicked. When the cursor leaves the popup window, the crosshair shape disappears and still does nothing.

EXPECTED RESULT
The cursor turns into a crosshair shape and picks a color when the user clicks any spot on the screen.

SOFTWARE/OS VERSIONS
Operating System: Artix Linux
KDE Plasma Version: 5.25.0
KDE Frameworks Version: 5.95.0
Qt Version: 5.15.4
Kernel Version: 5.17.15-hardened1-1-hardened (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION - WORKAROUND
The standalone Color Picker applet works just fine, so I used it to pick a color from my screen, then pasted the hex code into the Color dialog in System Settings, and changed my accent color in that way.