Quote Originally Posted by Q.G. View Post
The other side in this claims that yellow is an additive or subtractive colour, the result of a combination of two other colours.
You see what happens when those two colors are combined in additive system. Red and Green make Yellow. Do you deny that happens?
In a subtractive system yellow is already there.

No one on "the other side" ever said that yellow is not the color expression of a single wavelength, too. Only that the color can be perceived by combining Red and Green.