While you are right, "magenta" is just a fancy synonym for "purple". It might have been chosen instead of "purple" because the traditional word could be applied to colors having various proportions of red and blue, while "magenta" is intended to convey that the amounts of red and blue are equal. However all the traditional color names, like "red", "green" or "blue" refer to wide ranges of hues, not to a precise hue, so there was really no good reason for the use of the word "magenta".

"Cyan" is a very bad word choice caused by confusions in the translations of Ancient Greek texts made by philologists ignorant of chemistry and mineralogy. In Ancient Greek, "cyan" meant pure blue, not blue-green. More precisely, it was the color of the ultramarine blue pigment, the most expensive blue pigment at that time, which was imported from the present territory of Afghanistan and for which the name "ku-wa-no" was already used by the Hittites, a millennium before the Greeks. Nowadays ultramarine blue is still used as a pigment, but it is made synthetically, so its cost is a small fraction of what it was before the 19th century.

Before the use of "cyan" has started, the color name "blue-green" had been used for a very long time. Similarly, "orange" is a relatively new English word, but the color had been mentioned for many centuries, as "red-yellow" or "yellow-red".

So the awareness of distinct hues is not necessarily limited to the set of simple color words, because most languages have used compound words to name the hues for which they did not have a simple word.

Other languages have used the names of well-known colored objects to distinguish the hues that did not have distinct names. For instance, in Latin the word for "red" was used for both red colors and purple colors. When Latin speakers wanted to specify whether something was red or purple, they would say "red like the kermes (red) dye" or "red like the purple dye" (the word "purple" as a color name comes from the latter expression). Similarly, in Latin the word for green meant either green or blue-green. To distinguish the 2 colors, a Latin speaker would say "green like grass" or "green like leaves" or "green like emeralds" for expressing "green" and "green like the littoral sea" or "green like beryls" or "green like turquoise gems" for expressing "blue-green". So they were well aware about the differences between these colors, even if they did not have distinct words for them.

You have it backwards. Purple is a fancy synonym for dark magenta.

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