Pink

 Somebody on Reddit said "the only reason Indigo is listed as one of the colors of the spectrum is because Newton was obsessed with the number 7 for occult reasons" so I got to thinking, what names do people actually use for the fully saturated colors? So I went back to the xkcd color survey (the best resource available for questions about "what names do people actually use for colors") and looked at the fully saturated colors (the outer edge of this shape, not including the two edges that go to black):

There are a few extra colors on there beyond what we would normally name as the spectrum-- light blue, sky blue, cyan, and lime green-- and no indigo or violet, which always struck me as poor names for pure spectral colors-- but what interested me today was pink. Pink labels the whole region of the spectrum between red and magenta. I tend to think of pink as being a mix of red and white, but this region has no white at all!

This brought to mind some research I did last year about pink.

My friend Genevieve mentioned that a book she was reading said that pink was originally thought of as a shade of yellow. This, of course, is absurd to us, so we tried to figure out what they were thinking.

Here are some relevant points I found on Wikipedia:

"champagne" is listed as a shade of both pink and yellow:

https://en.wikipedia.org/wiki/Category:Shades_of_yellow

https://en.wikipedia.org/wiki/Category:Shades_of_pink

Apricot is listed as a shade of yellow, though it is closer to pink to my eye.

"Stil de grain yellow" was apparently originally called "pinke." A Book of Drawing, Limning, Washing (1652) says that you can mix it with blue to make green. This seems like the name pink (or pinke) was just used for another color originally.

Pink stretches a long way towards magenta, light purple, and beige in the shades of pink.

"The color pink is named after the flowers, pinks, flowering plants in the genus Dianthus, and derives from the frilled edge of the flowers. The verb "to pink" dates from the 14th century and means "to decorate with a perforated or punched pattern" (possibly from German picken, "to peck")." The only use I've ever heard of this in modern English is "pinking shears"-- scissors that cut a jagged pattern instead of a straight line.

This page does some very thoughtful analysis of the question of ancient primary colors:

https://www.handprint.com/HP/WCL/color6.html

A quote from that page:

The clearest statement of the proportions of light and dark that produce colors appears in Aristotle's discussion of the rainbow (On Meterology):

White [bright, pure] color through a dark medium or on a dark surface (it makes no difference) looks red. We can see how red the flame of green wood is: this because so much smoke is mixed with the bright white firelight; so too, the sun appears red through smoke or mist. ... When the sight [light] is relatively strong the [color] change is to red; the next stage is green; and a further degree of weakness gives violet. No further change is visible, but three completes the series of colors. ... The appearance of yellow [in the rainbow] is due to contrast, for the red is whitened [lightened] by its juxtaposition with green. ... Bright dyes too show the effect of contrast. In woven and embroidered fabrics the appearance of colors is profoundly affected by their juxtaposition (purple, for instance, appears different on white and on black wool). [374b]

These are almost the only colors which painters cannot manufacture, for there are colors which they create by mixing, but no mixing will give red, green or violet. [372a]

Aristotle apparently preferred the rainbow or veiling media as examples of "natural" colors because they represented a "pure" display of color variation; paint mixtures just muddled different colors together. This preference was maintained by the later Peripatetic philosophers who taught at the Lyceum. Thus, the De Coloribus advises that:

We must not proceed in this inquiry by blending pigments as painters do, but rather by comparing the rays reflected from the aforesaid known colors [white, yellow, and black], this being the best way of investigating the true nature of color-blends. ... [Thus], the different shades of crimson and violet depend on differences in the strength of their constituents, while blending is exemplified by the mixture of white and black, which gives gray. [792a-792b]

 Suppose someone invented a color theory based on watching the sunset.

The brightest color, pure sunlight, is white.

As it gets near sunset, the sun and surrounding clouds look yellow, then orange, then red.

After the sun sets, the night sky is blue, then black.

(Also, in terms of luminance, yellow is brighter than red is brighter than blue.)

So you have a line of colors from white to black that goes in order through yellow, red, and blue. This is what Chalcidus describes in 325 AD. The only trouble is what to do about green? Francois D'Aguilon puts it off to the side, as a mix of yellow and blue. But where is pink on this line? It has to go somewhere between red and white, which is yellow.

Along the same lines, you could also look at urine colors against a porcelain bowl-- White, light yellow, yellow, orange, orange-red, maybe even red if there is blood in it. So where is pink? Halfway between white and red: which is yellow.