Where has "invert colors" gone in the new MS Paint?
Solution 1:
Right-click!
Actually, the option still exists, but it's not found on the ribbon. You can only find it when right-clicking an image:
The result:
If you select everything or nothing, the entire image is inverted. Else, only the selection gets inverted:
The result:
Solution 2:
You can use the keyboard shortcut Ctrl+Shift+I.