Quote Originally Posted by JPD View Post
So one can say that a yellow-green shade is just that, and is not a mix of anything?
Indeed.
There is a band of wavelengths we call "green". And there is a (more narrow) band of wavelengths we call "yellow". (And other parts of the spectrum we need not concern ourselves with now).
And there is a transition area which we would call neither "green" nor "yellow". Or both, depending on our mood and what we ate yesterday.
That area is not a mix of "green" and "yellow". Just different wavelengths, different colours we do not have a proper name for.