English has different words for those two colors, too, "blue" and "cyan".

Languages have many words, but some of them are more... native... then others. How often do non-specialists use the word? How many 10-years old kids would recognize the word?

If you ask 10-years old kids, on a sunny day, what color is sky, I believe a typical Russian kid would say "голубое", but a typical American kid would say "blue" (not "cyan"). I think so; neither is my first language.

EDIT: Also, put "blue sky" in Google Translate. Then do a reverse translation; it's "blue sky" again.

So, the thing is that English technically has the word "cyan", but regardless, the American sky is blue; only the ink in printers is cyan.

"Cyan" isn't a basic color term in English; English speakers ordinarily consider cyan to be a variant of blue, not something basically separate. Something that is cyan could also be described in English as "blue". As opposed to say, red and pink -- these are both basic color terms in English; an English speaker would not ordinarily refer to something pink as "red", or vice versa.

Or in other words: Color words don't refer to points in color space, they refer to regions, which means that you can look at how those regions overlap -- some may be subsets of others, some may be disjoint (well -- not disjoint per se, but thought of as disjoint, since obviously you can find things near the boundary that won't be judged consistently), etc. Having words "blue" and "cyan" that refer to two thought-of-as-disjoint regions is pretty different from having words "blue" and "cyan" where the latter refers to a subset of the former.

So, it's not as simple as saying "English also has a word cyan" -- yes, it does, but the meaning of that word, and the relation of its meaning to that of "blue", is pretty different. These translated words don't quite correspond; we're taking regions in color space, and translating them to words that refer to similar regions, regions that contain a number of the same points, but not the same ones.

The bit in the comic about "Eurocentric paint" obviously doesn't quite make sense as stated -- the division of the rainbow doesn't come from paint! -- but a paint set that focused on the central examples of basic color terms of a particular language could reasonably be called a that-language-centric paint set. In any case the basic point is just that dividing up color space into basic color terms has a large cultural component to it.

"Eurocentric paint" is an imprecise phrase. I first read it as meaning "traditionally-used European paints", with the implication that other cultures chose their colors based on different paints. But the rest of the post makes clear it's the idea of basing colors on paints that's allegedly Eurocentric; so the better phrasing might be "Eurocentric fixation on paint".

I was taught in (US) school that the primary colors were red, yellow, and blue and the secondaries were green, orange and purple (which matches the 'rainbow' in the comic, though the 'rainbow' I learned was ROYGBIV).  Per https://en.wikipedia.org/wiki/Color_theory#Traditional_color_theory, this only works with paint:

One reason the artist's primary colors work at all is due to the imperfect pigments being used have sloped absorption curves, and change color with concentration...  Another reason the correct primary colors were not used by early artists is they were not available as durable pigments. Modern methods in chemistry were needed to produce them.

Granted, I was taught those colors in conjunction with being given paint to play with, which is a good reason to teach them. But it's still a bit striking that at no point in my education was I taught any other set of primary colors, except implicitly by picking RGB colors in MS Paint (an ironic name, in context).

I'm pretty sure that the common intuition among my classmates, way back in childhood, was that the first-tier colors were red, yellow, blue and green.  This turns out to be supported by a relatively sophisticated color theory based neither on natural occurrences of colors nor on any means of producing colors but rather the brain's fundamental abstractions for processing them.