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://thomasmansencal.substack.com/p/the-apparent-simplicity-of-rgb-rendering

The primary goal of physically-based rendering (PBR) is to create a simulation that accurately reproduces the imaging process of electro-magnetic spectrum radiation incident to an observer. This simulation should be indistinguishable from reality for a similar observer.

Because a camera is not sensitive to incident light the same way than a human observer, the images it captures are transformed to be colorimetric. A project might require infrared imaging simulation, a portion of the electro-magnetic spectrum that is invisible to us. Radically different observers might image the same scene but the act of observing does not change the intrinsic properties of the objects being imaged. Consequently, the physical modelling of the virtual scene should be independent of the observer.