Magenta has no spectral wavelength — it is the brain's response to simultaneous red and blue stimulation

Every point on the curved boundary of the CIE chromaticity horseshoe corresponds to a single pure wavelength. The straight ‘line of purples’ connecting the red and blue ends has no corresponding wavelengths — magenta and purple do not exist as spectral colors. They are created only as mixtures and perceived when both L (red) and S (blue) cones are activated while M (green) is relatively quiet. This is a fundamental fact: color is a brain construction, not a direct measurement of physics.

Examples

Spectral yellow at 570 nm is real. Mixing 700 nm red + 435 nm blue light produces a purple that has no spectral counterpart.

Assessment

Is magenta a ‘real’ color in a physical sense? What makes it appear in human perception, and why does it not appear in a rainbow?