The conventional understanding of opponent colors has red and green as one axis and yellow and blue on a second axis.
This perceptual opponency is a result of the trichromatic nature of human color vision in combination with subsequent
processing in the visual system. This red-green and yellow-blue opponency is fundamental to many different color
spaces. CIELAB, CIELUV, CIECAM02, IPT, YCC and more all incorporate this concept of chromatic opponency. In
most cases the yellow and blue opponent axes are reasonable. However for the red-green axis it is more like a purplegreen
axis due to a consistent, significant bending of the red-green axis. Is dark purple the opposite of green? This paper
summarizes the result of analyzing a wide range of color spaces based on their actual opponency. The consistent
limitation of a shared matrix formulation for opponency is discussed and finally a simple, invertible color space is
considered. The angular differences between quadrants and computed antonyms is shown to be significantly more
consistent using this hypothetical alternative color space.
Nathan Moroney, Nathan Moroney,
"The opposite of green is purple?", Proc. SPIE 7241, Color Imaging XIV: Displaying, Processing, Hardcopy, and Applications, 72410N (19 January 2009); doi: 10.1117/12.817013; https://doi.org/10.1117/12.817013