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This illusion forces your brain to see color in a black and white photo. It works by first saturating your visual memory with shade and line data, which occurs when you stare at the first picture for more than ten seconds or so. Upon flipping to the second picture, the spectral opposite of the original color data is imposed on the grey lines and shading of the photo.
If you are curious about how afterimage works, our eyes essentially get fatigued of the color we are staring at and, seeking neutral, start to emphasize the complementary color. Essentially, the dark blue in the woman’s skin is the complement of her light yellow white skin color.