I discovered this weird trick while I was playing the demo for Giants:Citizen Kabuto. I was too lazy to read the key map for the game so I started experimenting, and suddenly the screen colors became inverted. I thought at first that the colors were only in the game, but when I quit the game, my desktop's colors were inverted also. I found the cause this to be a "com.Apple.CoreGraphics.plist".
If you want to play with this wierd glitch, create a preference file named "com.Apple.CoreGraphics.plist". Name the child "DisplayUseInvertedPolarity". Set the type to Boolean and the value to "yes". To get Aqua back to normal just change the value to "no". It gives a different look to all your applications. For some reason any screenshots I've taken with Grab.app look normal, so you can't exactly show this off very well without trying it yourself.
[Editor's note: At first, I thought this was just the "Inverted screen" effect from the Universal Access preferences panel, but it's not. Things are still in color, but they're all the polar opposite of their original colors. It's quite the interesting effect, and none of the screen capture tools will reflect the image. So I tried with a digital camera; here's a small snippet of an inverted polarity screen:

defaults write com.apple.CoreGraphics \The backslash should allow you to copy and paste the above in one step, but in case it fails, it needs to be on one line. You'll have to logout and login to see the effect. To disable it, you can repeat the above with "NO" at the end, or just delete the com.apple.CoreGraphics.plist file in ~/Library -> Preferences.]
DisplayUseInvertedPolarity -bool YES
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021205043933844