First look at a yellow dot on an white square for up to a minute. Then look at a black square. The result should be a dot of the color 'Stygian blue'; the picture shows an approximate rendering of what it's like. This is part of a picture made by Zowie and put on Wikicommons here: https://en.wikipedia.org/wiki/File:Chimerical-color-demo.svg

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   John Carlos Baez (johncarlosbaez@mathstodon.xyz)'s status on Sunday, 14-Jul-2024 22:30:34 JST John Carlos Baez

   𝗦𝗧𝗬𝗚𝗜𝗔𝗡 𝗕𝗟𝗨𝗘

   What color is the river Styx - the river that the dead must cross to reach the underworld in Greek myths?

   'Stygian blue' would be a good name for it. But Stygian blue is actually what you see by staring at this yellow dot for a long time and then looking at the black square. It's a weird color: somehow simultaneously deep blue and black.

   Stygian blue is an example of a 'chimerical color'. This means it can only be seen by looking steadily at a strong color until some of your cone cells become fatigued, temporarily changing their behavior. If you then look at a new color, an afterimage with the color opposite to the one you were staring at gets overlaid on the new color.

   Chimerical colors are fun because they can't be explained by the usual simple theory of vision! In the simple theory, each of the 3 kinds of cone cell is getting activated by some amount, so you just need 3 nonnegative numbers to describe a color. To explain chimerical colors, people resort to more complex theories of vision, like these:

   https://en.wikipedia.org/wiki/Opponent_process