I left the Mac open while helping Mrs Piper in the kitchen - came back and the colours are all reversed, like an old colour negative. PiperCub must have been tinkering.
Any idea how to get the colours back to normal? I re-started, but it didn’t make a differences.
Try control-option-command 8
Worked - thanks much!
Doesn’t work in 10.8. You have to go to Preferences -> Accessibility -> Display and check the “Invert colors” checkbox.
Why would people want to invert the colours? Why is this available under “Accessibility”?
yes - why is there this option? and how did you know the command, Bag of Mostly Water? seems very obscure.
I found a discussion on the Apple boards addressing this.
Thanks!
It appears from that discussion that this function is primarily used by young’uns. 
I think it’s an attempt to increase contrast. White on black seems to have a slightly higher contrast than black on white. Still, I’d agree it’s mostly useless without also changing other colors to make them high contrast.
And no one benefits form seeing an image in negative. Just make it bright.
From Wikipedia:
This is confirmed by a friend of mine who works with visually-impared people.
Ah, that makes sense. I’ve heard before that text is easier to read if it’s white on black; didn’t occur to me when I asked for help, because the app I had open when the Cub worked his magic was full of images.