Anybody who claims magenta isn’t a color is stupid, lying, or link-baiting. Take it from a color-blind person: all colors are a matter of perception, and claiming Magenta isn’t a color because it doesn’t fit neatly in the linear spectrum of visible electromagnetic radiation is like saying this isn’t music because the vibrations that tickle our ear aren’t the result of a monotone sinusoidal wave.
We have no equivalent of polyphony for light, but just as it took a whole orchestra to make Jaws scary, the colors we perceive are most commonly a mixture of different frequencies of light. Magenta is simply what we see when when we see light from both ends of the spectrum at once.
Here’s a better question: where is brown in the rainbow?