If that fails, go to System Preferences > Accessibility > Display and check the box titled 'Invert Colors'.
Did that scare you ? Don't worry. It's completely reversible and completely harmless. Pressing the same keys simultaneously will revert the colors back to normal again. Alternatively, uncheck the 'Invert Colors' box to revert back to normal colors again.
Additional Tip: You can use this when in Photo Booth on Mac to avoid the blinding white-screen flash that comes every time you take a photo.