Make your Mac run in Grayscale mode
You can run Mac OS X in Grayscale mode by adjusting the display settings in the “Universal Access” control panel. Likewise, you can stop a Mac from running in greyscale mode and get full color back by disabling the setting in the same system control panel.
The settings are part of the standard Universal Access capabilities inside System Preferences, and while they are intended to be used for people who are visually impaired, I have seen a few pranksters adjust Mac’s to grayscale mode just for fun – it certainly confuses people.
Note that changing your Mac to run in grayscale mode does not actually alter any graphics or images to be permanently black & white.