This is another version of something I came up with back in 2006. It was originally called the Candy Stripes Illusion. Here’s more of a spiral version of the same thing, with slightly different colors.
The illusion is simple to explain, and by being a spiral, it’ll automatically adjust to different people’s eyes…. The green is the same throughout the image, although when it is surrounded by red or blue, it seems slightly different. By adding the spiral effect to it, you can look to the outside of the image and the illusion isn’t that strong. However – as you look about three spirals deep (not the circles, but the spirals) it will start to look as if there is a fourth color in the picture – a slightly different shade of green.