The brilliant Phil Plait, an astronomer who writes the blog Bad Astronomy, posted this yesterday and I am amazed.
You can see embedded spirals in this image in green, pink-orange and blue. Well, wonder of wonders, the green and the blue are actually the same color. Phil popped the original image into Photoshop and isolated the "two" colors to be sure. Here they are side by side.
Sure enough, they are both RGB 0, 255, 150.
Human brains are amazing machines, but cameras they are not. I deal with identifying colors for a living and I'm pretty aware of the tricks my brain can play when I'm trying to judge what color a specific color is. I always look at colors in isolation when I can and have been known to mask off whole sections of carpet patterns so that I can really see the color I'm looking at. I warn people all the time about looking at paint chips in a fan deck --don't do it! Your brain can't judge with any degree of accuracy a color that's displayed in a line up of similarly hued and tinted colors.
So let's look at what's going on with this fantastic optical illusion.
These look like different colors because this illustration is shortchanging our brains' ability to contextualize these "blue" and "green" colors. As you can see in the close up, the magenta and orange stripes aren't continuous. The orange stripes don't go through the "blue" spiral and the magenta stripes don't go through the "green" one.
Without any conscious awareness of it, our brains are forced to see the "green" stripe in contrast with the orange stripe and the color reads as green. Similarly, the same "green" color, when we're forced to compare it to magenta reads as blue. To prove this, concentrate on the spot where the "blue" stripe butts into the orange stripe on the right side of the image. If you stare at it long enough it will read as the same green as the stripes on the left side of the image. But only at the point where the two colors intersect. It's wild! If I stare at that intersecting point, my brain will let me see the green but only at the intersecting point. If I widen my focus slightly, it still looks blue in the center of the stripe.
Human brains are amazing things but they cheat and play fill in the blanks all the time. As a designer, I think I'm pretty aware of this and I'm pretty adept at forcing perspective and hiding flaws. Usually, I'm pretty good at seeing though these kinds of optical illusions. I have to say though, that this one surprised me. I love this kind of stuff!