The February issue of SciAm has an interesting article about our color perception. It describes visual processing based on opposing colors: Red versus green & blue versus yellow. This process has been accepted for over 100 years as an explanation for various characteristics of human color vision. One aspect of the process is the absence of hues like reddish-green & bluish-yellow. These are impossible (or forbidden) colors for the human visual system, although the article describes experiments resulting in some capability for seeing such hues. Note, of course, that various other combination hues are easily seen: Bluish-green, reddish-blue (lavendar, purple), & reddish-yellow (orange).