Just the other day I was going through some Remote Sensing books, preparing a class on color theory. It is just cool the way our brain works and how it changes our perception of colors. Our brain always try to identify the color (or shade of gray) by comparing it with the colors/shades in its surroundings. This way we always tend to see a shade of gray a little darker than it really is, and if someone would ask you to point the 50% mark on a gray scale gradient, you would most likely point to the 60 or 65% shade.
Another really cool illusion happens with colors. See this image:
What do you see? Spirals of green, pinkish-orange, and blue? NO! The ‘green’ and the ‘blue’ spirals are of the same color!!!
In this one, Dr. Phil Plait (Bad Astronomy) show us a zoom, where you can see that the stripes are not continuous:
And in this other one, Richard Wiseman used photoshop to change the colors outside the spirals into black:
Amazing, right? So remember that your brain is always working when you’re seen things. Visual experience is subjective, and you past experiences influence on it.