If the user does not select a image for the Desktop, then (at least in High Sierra)
NSWorkspace.shared.desktopImageURL(for: NSScreen.main!)
returns something like
file:///System/Library/PreferencePanes/DesktopScreenEffectsPref.prefPane/Contents/Resources/DesktopPictures.prefPane/Contents/Resources/Transparent.tiff
which makes sense- a transparent tiff.
How do i determine the color the user selected for the Desktop?