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I am aware that magenta is not a 'real' (i.e. spectral) colour and is a result of stimulation of both the blue and the red cones in our eyes. However, when the red and green cones are stimulated (by red and green light), we get yellow -- a colour that can indeed be found on the spectrum and which lies between red and green.

  1. When combining red with green, how come we don't get a new, non-spectral reddish-green like we get with red and blue?
  2. Conversely, when combining red and blue, why do we get a non-spectral magenta instead of green -- the colour that lies between blue and red?
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Cones are not red, green and blue receptors, and they detect color over broader ranges of wavelength. A color space used to model cones in human vision is LMS space.

In the LMS model The L component is essentially "blue". However the M and S components are actually very closely overlapping with the S being being at the red end and the M being the "logical" green in the middle.

The colors you "see" are an interpretation by your brain's vision system of all this data from receptors. It is a 3-D color space with three coordinates and the colors you "see" are based on that map, not on the simple 1-D line of frequency. Your brain is not even trying to map to frequency/wavelength, it uses it's own internal 3-D color space.

With that in mind :

  1. Mixing red and green means that stimulates (mostly) the S and M cones. Now think about what a yellow light would do : it would stimulate the S and M cones in a manner similar to a red and green combined source. So your brain needs to identify yellow and a by-product of that is how it interprets red-green mixes to be consistent.

  2. Red and Blue light does not just stimulate the L and S cones it also stimulates the M cones (because the M cone overlaps the S cone a lot in coverage). Your brain now has three different inputs essentially saying it has a mix of all three receptors. Your brain, however, doesn't care about wavelengths and uses a three point coordinate system (or seems to) to label colors. The label it uses is what we call magenta.

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