What color are these skulls? (Hint, they are not orange or purple)

If you're like most people, you see two different colored skulls above. But they're actually the same color. And no, the color is not purple or orange. It's actually red.
How can that be? It's called the Munker-White Illusion (or Munker Illusion, explained here by Gizmodo).
Via Popular Science:

The pigments morph because of the ­Munker-​White illusion, which shifts the perception of two identical color tones when they’re placed against different surrounding hues. No one knows for sure, but the illusion probably results from what David Novick, a computer scientist at the University of Texas at El Paso, calls the color-completion effect. The phenomenon causes an image to skew toward the color of the objects that surround it. In a black-and-white image, a gray element would appear lighter when it’s striped with white, and darker when banded with black.
Many neuroscientists think that neural ­signals in charge of relaying information about the pigments in our visual field get averaged—creating a color somewhere in the middle. Here, one skull is covered by blue stripes in the foreground and the other with yellow ones. When the original skulls take on the characteristics of the separate surroundings, they look like different colors entirely.
Here's a visual that shows us how it works:

Image: Hubert Tereszkiewicz
See the full image at Popular Science