Mixing colors

Mixing light of red and green wavelengths produces something that appears yellow because of how the human visual system works.

But, that mixture does not actually have light of yellow wavelengths, even though these two cases appear (to us) to have the same color.

 

Redbelly eloquently explains it. If you did a spectrographic analysis (or simply passed it though a prism) you would see that the red and green light is still there, and that there is, in fact, no yellow light at all, except in our minds.