Build Colors from Colors with CSS Relative Color Syntax

Not on topic at all, but crazy coincidence to get so many “c” words on my left hand side: https://imgur.com/a/thwKg0b

I’m not sure justified text works well here, lots of “rivers” through the text.

We -- technical people anyway -- usually associate a given color with a particular frequency of light. But I read somewhere that the color brown results from the addition of two (or more?) frequencies of light. Is that true?