This image posted at reddit is an illusion. The twelve circles appear to be four different colors, but they are actually all the same color. You can enlarge the image here, which will help you see the real color of the circle. It’s kind of peach. They appear different due to the different colored stripes allowed to cross them. What we have is an example of a Munker Illusion.
Yes, there are some people that the illusion will not work on. You can see a video that explains the illusion further at Digg.