Basically, how do we differentiate between different colours and different shades of the same colour.
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As people said its because we agreed to name them. My favorite one is the red robin, or redheads. They are both clearly orange. But there was no name for orange yet. So now we still call them red.
“Pure” blue and green are different wavelengths.
You can get each shade either by emitting light of that wavelength or by mixing colors, because in the end, human eyes “measure” color with three kinds of sensors.
Let’s say the green sensor gives a reading of 10 for green light, 2 for blue light, and 7 for turquoise. The blue sensor gives a reading of 10 for blue, 2 for green, 7 for turquoise.
The eye will see a “pure” turquoise as (7, 7), and it will see a mix of dimmer (let’s say 58% as bright) blue + green light as:
* Green sensor: 58% * 10 = 5.8 from the green light + 58% * 2 ~= 1.2 from the blue light -> total of 7
* Blue sensor: the other way around, also 7
-> looks the same.
We can with various tricks distinguish this (eg. by splitting the light with a prism or refraction grid – the rainbow reflection of a CD splits light like this).
“Dark blue” vs. “light blue” means (if it’s the same color) we get it by having more or less light of the same wavelength.
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