If anyone's wondering why this works, it's because the eye gets tired (i guess that's what to call it) because it's seeing the same color for a long time. Whenever the color disappears, our eye sees the opposite color (the color on the other side of the color wheel) for a brief amount of time. This only really works well outside of our peripheral vision, so you only see a green dot when you are looking away from the "green dot."
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