Grey? Blue? Purple? It can look different, depending on the context. Let's talk about color perception, color temperature, and the history of laundry.

Atmospheric opacity image from ESA/Hubble (F. Granato): https://www.eso.org/public/images/atm_opacity/ - image licensed under CC by 4.0 - https://creativecommons.org/licenses/by/4.0/

The optical illusion is original, but based on the work of David Novick: https://twitter.com/novickprof/status/1139342022551191553

More on Illuminant D65: https://en.wikipedia.org/wiki/Illuminant_D65 - the references in this article go into the full technical detail

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