https://www.msn.com/en-us/news/world/secret-of-ancient-maya-blue-pigment-revealed-from-cracks-and-clues-on-a-dozen-bowls-from-chich%C3%A9n-itz%C3%A1/ar-AA1EfJpq

Maya blue is a highly unusual pigment because it is a mix of organic indigo and an inorganic clay mineral called palygorskite.  

Echoing the color of an azure sky, the indelible pigment was used to accentuate everything from ceramics to human sacrifices in the Late Preclassic period (300 B.C. to A.D. 300).

A team of researchers led by Dean Arnold, an adjunct curator of anthropology at the Field Museum in Chicago, determined that the key to Maya blue was actually a sacred incense called copal.
By heating the mixture of indigo, copal and palygorskite over a fire, the Maya produced the unique pigment, he reported at the time.

https://x.com/AkiyoshiKitaoka/status/1798705648001327209

The left face appears whitish and the right one blackish, but they are made up of the same luminance.

https://community.wolfram.com/groups/-/m/t/3191015

Illusory staircase Gelb effect 
https://www.psy.ritsumei.ac.jp/akitaoka/illgelbe.html

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Björn Ottosson proposed OKlch in 2020 to create a color space that can closely mimic how color is perceived by the human eye, predicting perceived lightness, chroma, and hue.

The OK in OKLCH stands for Optimal Color.

• L: Lightness (the perceived brightness of the color)
• C: Chroma (the intensity or saturation of the color)
• H: Hue (the actual color, such as red, blue, green, etc.)

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