Perception of color by the human eye is surprisingly dependent on colors surrounding the object in question. All of the hearts in the checkerboard above are exactly the same cyan color, even though they appear to be shades of green and blue. "The image, by Kitaoka, is based on the dungeon illusion discovered by vision scientist Paola Bressan of the University of Padua in Italy."
The same principle applies to the pair of wolves in these figures. When the graded-color background is removed, their identical coloring is obvious - but even in retrospect it is difficult to discern that similarity in the situation on the left.
These two illusions are samples from a set of 26 assembled at Scientific American for their Illusions: Colors Out of Space slideshow.
Link.
The compression on the wolves image have made the wolves on the gradient background sightly different in color than those on the right. However, it is not as dramatically different as with the hearts.
There is more info on the creation of such illusions at this link:
http://www.psy.ritsumei.ac.jp/~akitaoka/color12e.html
Oh well, the dots are sort of close enough to tell that they were probably the same colour before compression.
Other things:
#5 -- doesn't work for me at all. Does anyone see a "red" ring on that one? It just looks like different shades of blue to me.
#15 -- doesn't work for me at all.
#16 -- doesn't really work for me, I can kind of see the neon lines after I'm told to visualize them there...
#25 -- I can say the colors or read the words without any confusion whatsoever (first try). I wonder how much of these visual anomalies is a matter of conditioning and not actually fundamental to the way the brain processes images.
#26 -- SOOOO not worth it. It is kind of impressive that the illusion lasts for a long time, on the other hand minutes later I still have lines and stripes embedded into my vision wherever I look. This effect is pretty subtle, not dramatic at all (for me at least), so don't expect anything great from putting in the six minutes to stare at that, it will just hurt your eyes, and the only impressive thing is that the effect lasts for a while.
http://izismile.com/2009/06/18/cool_optical_illusion_1_gif.html
There's plenty of others like it out there but I like the caption at the bottom of this one.