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Magenta is a color made up of equal parts of red and blue light. This would be the precise definition of the color as defined for computer display (the color #FF00FF shown in the color swatch above). It is a pure chroma on the RGB color wheel (Image of RGB color wheel:) midway between violet and rose. In HSV color space, magenta has a hue of 300°.
Magenta is not a spectral but an extraspectral color: it cannot be generated by light of a single wavelength. Humans, being trichromats, can only see as far as 380 nanometers into the spectrum, i.e., as far as violet.