Let’s say your Mac should display a screen like this:
But instead it looks a bit more like this:
Somehow, the “show negative colors” preference has become activated. This is a preference through the Universal Access preference pane that allows people who have difficulty reading black text on a white background (or vice versa) to instantly switch the way the colors are displayed.
This preference is both activated and deactivated by the key combination control-option-command-8. Note that some long-term Mac users will refer to the command key as the “Apple” key, since on older Macs it had a little Apple logo on it. By pressing the ctrl-opt-cmd-8 key combo, you will be able to get back to your regular color display mode.
How does this happen in the first place, you may wonder? When you look at your keyboard, you’ll see that the control, option, and command keys are right next to each other, while the 8 is far away. In theory, this makes it much more difficult to accidentally activate, but I’ve personally found that they make it very easy for a cat walking across your keyboard to activate them. I’ve also had at least one co-worker accidentally activate it by dropping a huge pile of papers on her keyboard.