Incredibly, the blue and the green spirals are the same color. From the article: The orange stripes go through the "green" spiral but not the "blue" one. So without us even knowing it, our brains compare that spiral to the orange stripes, forcing it to think the spiral is green. The magenta stripes make the other part of the spiral look blue, even though they are exactly the same color.
Don't believe it? Load it in Photoshop and perform color sampling.