An Easy Question,

but with an unexpected and unexplained answer,

the effect is known, and is called the Munker-​White illusion, but it is still unclear as to what causes the effect, back to the illustration above, what do you see? two red skulls of course! and they are, both red, it is just that your mind refuses to see them, you are fooled into believing you are looking at a purple skull with a orange one next to it, the Munker-​White illusion, only the striped background changes our perception of their color,

here is an even better example of it, remember the two circles are both green, and stay green during the entire video, 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 why 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, amazing! skull image credit, Popular Science.