I'm wondering where the old "invert colors" feature from MS Paint has gone. My new version of MS Paint 6.1 doesn't seem to have this feature?
Actually, the option still exists, but it's not found on the ribbon. You can only find it when right-clicking an image:
If you select everything or nothing, the entire image is inverted. Else, only the selection gets inverted:
You can use the keyboard shortcut Ctrl+Shift+I.