When you click on this link, you’ll see an image of a castle in greyscale. Click on the picture, it will change, and you’ll see a small black dot. Stare at this dot for about 20 seconds, then while keeping your eyes on the dot, click the picture again.
I saw that one a few weeks back. It was neat for the first few seconds, but once I realized it was just using complimentary colours (which become inversed as an after-image once you turn your eyes away and refocus on the picture) it sort of lost its magic. The “misdirection” in this magic trick is that the castle doesn’t change; it only looks like it’s in “colour” (despite being gray stone in the first place) because the surrounding scenery is.
Well, that’s what I meant – just the appearance of the change from the afterimage melding with the actual colours to appear to transform the image into its proper natural colour. It’s a neat effect and probably the ultimate demonstration of the peculiarities of vision.
The castle though – I can’t really tell if the apparent change is a result of the apparent changes around it giving it the illusion of “real” colour, or if the castle’s complimentary is subtly but importantly differtent enough to give it that “natural” look.