The sky is only blue because we call it blue. In fact, humans didn't have a concept of "blue" for millennia. The ancient greeks most commonly called the sky "red" because there was no name for "blue".... mostly because blue isn't a very natural color; it is extremely rare in nature, and for most of human history, could only be reproduced by crushing gemstones (lapus lazuli) or certain beetles (the beetles didn't make good, consistent dies, I think). (it's why blue was traditionally the most expensive pigment for oil painting, and why in the works of the masters and other Renaissance painters, blue is only ever used in the virgin Mary. Exclusively)
In fact, the only reason humans now call the sky "blue," is because they are trained to from childhood. (color-object association translates to novel items but, for whatever reason, association of blue-to-sky doesn't translate unless you are coached to do it).
Crazy!
The historic pigments in the Forbes Collection include the esoteric, the expensive, and the toxic.
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