I think there is more to it than a simple "facepalm". In the modern RGB model, green and blue are primary colors, and mixed together, you get cyan, the opposite

I think there is more to it than a simple "facepalm". In the modern RGB model, green and blue are primary colors, and mixed together, you get cyan, the opposite of red. So the explanation is generally correct, the only thing wrong is that yellow is considered a primary color instead of green. But look here, and remember the book was published in 1864: https://en.wikipedia.org/wiki/Complementary_colors https://en.wikipedia.org/wiki/Complementary_colors Look at the color wheels of that period and earlier, especially this one: https://commons.wikimedia.org/wiki/File:Color_star-en_(tertiary_names).svg https://commons.wikimedia.org/wiki/File:Color_star-en_(terti... And you see that red is the opposite of green. Why not cyan? I don't know. I guess cyan simply wasn't a well recognized color back then, maybe cyan was called green (a blueish green). Maybe our perception changed with the availability of modern pigments and screens. As for the color you see when trying the illusion, it is even more interesting because it is actually an impossible color! [1] It doesn't correspond to a wavelength of light or a combination thereof, you can only perceive it through an illusion like this one. If we follow the Wikipedia article, and considering that the afterimage appears on a dark background that would be "stygian cyan". Spooky! Again, the theory was probably unknown in 1864. [1] https://en.wikipedia.org/wiki/Impossible_color https://en.wikipedia.org/wiki/Impossible_color