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.