Mixing Without Merging

Magenta appears when your eyes receive red and blue light at the same time with no green. This mix has no single wavelength, because red and blue sit at opposite ends of the spectrum with no physical bridge between them.

Your brain solves the problem by inventing a color the rainbow does not contain. It fills the gap between red and blue with magenta, keeping your internal color model consistent. The color is real to you, but it has no counterpart in the electromagnetic spectrum.