In case you're wondering, this is not just a spooky add-on from a maniacal Apple programmer. It is actually an Accessibility feature built-in to the Mac OS to assist with sight-impaired users.
For more Accessibility features check out: System Preferences > Accessibility
I love this one...it makes everything on the screen a negative. I like to use it for design ideas, and screenshots sometimes.
Hit Ctrl+Option+Command+8 and see the madness!