Visible light spectrum: http://upload.wikimedia.org/wikipedia/en/7/79/Spectrum4websiteEval.svg Wikipedia's article on color vision: http://en.wikipedia.org/wiki/Color_vision Response intensities for cone cells S,M,L: http://upload.wikimedia.org/wikipedia/en/1/1e/Cones_SMJ2_E.svg The way how a mixture of blue and green looks cyan, and how a mixture of green and red looks yellow, makes sense: The cyan wave lengths stimulate both S and M cells, so the eye cannot tell the difference between cyan, and a mixture of blue and green. The yellow wave lengths stimulate both M and L cells, so the eye cannot tell the difference between yellow, and a mixture of green and red. However, the way how mixture of blue and red looks violet doesn't make sense equally, because the violet is not in between blue and red in the spectrum. The only way I could explain that blue and red appear violet, is that the L cell must have two response intensity peaks. One at centered at the red wave lengths, and other centered at the violet wave lengths. I don't see anything about this in the Wikipedia's article, but curiously, my guess is not in contradiction with the diagram of the response intesities: The red curve stops before it reaches the left corner! What would happen, if the curve was drawn all the way to the end of the visible spectrum? Would the red curve make a new peak there? It looks like the person who has created the diagram must know that there is something strange happening with the red curve in the left corner, because otherwise he or she would not have been able to avoid making mistake, by avoiding to draw the curve all the way to the left, so carefully. I don't have other sources at the hand right at the moment. But I remember seeing a same kind of figure in some book too: The red curve is not drawn completely. The Wikipedia states: What does the "almost" mean there? If this claim is true, then why does the combination of red and blue show as violet to the human eye? And why isn't the meaning of the word "almost" made more explicit in the figure?