Perplexing Purple

We have three sets of cones (or colour sensors) in our retinas, each of which is sensitive to a different part of the colour spectrum; the brain then constructs the rest of the spectrum by extrapolating from the relative strength of these three. In the case of purple, which occurs when the red and blue sensors but not the green ones are triggered, the brain creates a colour to fill the gap. If your brain were more objective, rather than showing you purple, it would display a patch of flickering grey with the words “system error” on it. — Rory Sutherland in Alchemy: The Surprising Power of Ideas That Don’t Make Sense (2019), section 6.2