The release notes for build 6645 say:
Added: Missing shortcut for selecting images with no color class. Note: Windows Input Method switching may interfere with the functioning of the shortcut (Ctrl+Shift+0) and may have to be changed in the Keyboard Control Panel's "Input language hot keys.
I have been unable to make this work. I am on Windows 10. I have changed the "Switch Keyboard Layout" key sequence from Crtl + Shift to "not assigned". The "Switch Input Language" key sequence is set to Left Alt + Shift. So I need more detailed instructions as to how to make this work.
However, I have discovered a work around. I have found that Crtl + Shift + 9 will select all photos that have a colour class assigned. Therefore Crtl + Shift + 9 followed by Crlt + Shift + O (select others, oh not zero) will do what I want.