My mother has macular degeneration, so I was experimenting with the OS X grayscale feature.
This is one of OS X's accessibility features. Grayscale is a secondary option for what's mislabeled as the "White on Black" display. In fact the option isn't "White on Black", it inverts pixels to produce a negative image. So, yes, text is white on a black background, but colors are inverted to.
The default normal setting is "Black on White". You switch to grayscale with a checkbox. The alternate setting is "White on Black", which can also be switched to grayscale.
It's a bit of weird UI, and there's no keyboard shortcut for the grayscale option. That's a shame, as I suspect being able to toggle the standard display to grayscale would be more useful than the color inversion option.
There's a keyboard option for the inversion though. The default is "Command-Option-Control-8", though like all OS kb options it's easy to change to another combination. I'm sure kids use this feature in the school computer lab all the time.
The tip for non-visually impaired persons is that either the grayscale or the Black on White option both grealy reduce screen light output, and probably reduce battery drain.
It also brings back memories of the pre-VGA days on the early PC, when grayscale displays were a big improvement on the Hercules Graphics Card output. I once worked ran Mac OS Classic 7 with a massive > 100 lb grayscale secondary display, running OS X this way puts me back in grad school ...