But the point is, there is no line which separates white and black (or green and blue). 50% grey is neither black nor white, it's grey. Turquoise is neither green nor blue, it's turquoise.
But the point is, there is no line which separates white and black (or green and blue). 50% grey is neither black nor white, it's grey. Turquoise is neither green nor blue, it's turquoise.
I see it as having a blue component and a green component. If the mixture has more green than blue, then it's green.
The analogous version in black and white is "is this dark grey or light grey?" because that's the one asking you to guess which side of the 50/50 split the color is on.
Ok, but presumably you can make a test that goes from 50% gray to 100% black and you have to say "this is black" or "this is gray"
No scientific line. But where does your mind put it if asked without being told which it is? This test is about where you perceive that line to be.
but when does turquoise start and end and green starts and blue ends? or is there just another color there between them. And then what about that color?
I think you're (accidentally?) hitting on exactly the point there.
For some people's language usage, blue and green are adjacent colors, and thus defining a point that divides them is perfectly fine.
For other people, these are not adjacent -- for some people, there's a single color (aqua? turquoise?) between them, and green and turquoise are adjacent colors, as are turquoise and blue, and it's reasonable to ask about a dividing point between those adjacent pairs.
For those who don't use language this way -- do you consider red and blue adjacent, or do you consider purple (violet?) a necessary intermediate? Are you comfortable defining a point between red and blue, or are you instead comfortable defining a point between red and purple, and a point between purple and blue?
And for all I know, there are people for whom blue and green (or blue and red) have a distance greater than one, or greater than two...
Are there really people whose language treats “cyan” and “turquoise” as distinct colors which are not in the “blue-green” family?
I don't know if there are people who treat turquoise and aqua as distinct, but I certainly treat them as distinct from blue (azure, cobalt) and green. Several of the colors around the mid range in the linked page are not colors I would use the words "blue" or "green" for. That doesn't mean that I have strict rules here; I don't actually know if I would call what you call "cyan" turquoise or blue; ditto plenty of other words like "seafoam." That's kind of my point -- modulo another poster's comment about this being a test of bad monitor calibration, it's really more about language than about color.
I think there's another set of questions here -- why is "blue-green family" a thing in your mind, rather than "blue-yellow family"? Is there a "red-blue family"? "Orange-blue"?
Our green cones are the most sensitive and their range significantly overlaps with red cones, so it's only natural that going from green towards red you'll be able to make clearer distinctions between colors than the other way.
Also, yellow-blue and red-green are opponents that can't be mixed because of how our retinas preprocess the signals from cones. Therefore, you obviously end up with blue-green (cyan), red-yellow (orange), yellow-green (lime) and blue-red (magenta, which actually doesn't exist on the light spectrum) families of related colors.
Many Slavic languages do that, as well as Albanian for some reason?
Russian speakers broadly consider sky blue / turquoise / cyan a distinct color right between green and blue
https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
Although the question "is the color distinct and basic or just a shade?" is very subjective. Is pink distinct or a shade of red/purple? Is purple distinct or a shade of red/blue? Is green distinct or a shade of blue? (it's well-known that in Japanese green separated from blue only relatively recently, with very bluish traffic lights and other quirks included)