There’s a built-in accessibility option that can be very confusing to somebody who doesn’t know how to turn it off, and it requires only pressing one shortcut key sequence:
Shift + Alt + PrintscreenThat’s it. Once you press that key combo, their desktop will get flipped to the high contrast mode—you can press it again to go back to normal. The great thing is that even if their computer is locked, you can enable high contrast with the icon on the lower left corner. On Mac OS X, you can press Ctrl+Opt+Cmd+8 to invert the screen’s colors.
