The blue and the green

Phil Plait in Bad Astronomy:

Via my evil twin Richard Wiseman comes one of the best color optical illusions I have ever seen. The original was apparently posted on Buzzhunt:


You see embedded spirals, right, of green, pinkish-orange, and blue? Incredibly, the green and the blue spirals are the same color. At first I thought Richard was pulling our collective legs, being a trickster of high magnitude. So I loaded the image in Photoshop and examined the two spirals. In the two squares displayed below, the one on the left is colored using the same color from the blue spiral, and on the right using the green spiral.

green and blue squares

Like I said, incredible! For pedantry sake, the RGB colors in both spirals are 0, 255, 150. So they are mostly green with a solid splash of blue.

The reason they look different colors is because our brain judges the color of an object by comparing it to surrounding colors.

More here.