| The current use of "cyan" for blue-green is a modern confusion caused by people who have used Greek words without bothering to check their true meaning. In Ancient Greek, "cyan" was blue, not blue-green. More precisely, it was the color of the pigment "ultramarine blue", which has remained widely used until today. The name of this pigment was already used by the Hittites, long before the Greeks. An example of a Latin author who distinguished consistently green, blue-green and blue in many places is Pliny the Elder. Blue was referred to as the color of the sky or the color of the blue pigments used in painting, like ultramarine blue. Green was referred to as "green like grass", "green like tree leaves" or "green like emeralds". Blue-green was referred to as "green like the littoral sea", "green like turquoise" or "green like beryls". This is especially obvious in the discussion about emeralds and beryls, which are identical but for their color, the former being green and the latter blue-green. Similarly, in Latin "red" was used for both red and purple, but the two colors were distinguished as "red like crimson dye" (beetle-based dye) and "red like purple dye" (snail-based dye). |
This is a misunderstanding of how language works. Words don't have any "true" meaning. A word exists to convey an idea from a speaker to a listener. If at that moment the intended meaning is conveyed, that is the word's true meaning at that point in time.
When I say "cyan" and a listener pictures a light color whose hue is around 180° similar to the sky on a clear day, then neither one of us is confused and the correct information has been transmitted from one brain to another.
Whether some long-dead Greeks would have used that same blob of phonemes to convey a different spectral idea is irrelevant. When you said "turquoise", but did you mean to convey "from Turkey"? When you said "beetle", did you mean to convey "little biter" or an insect? Probably not.