Why can't you find magenta in the visible spectrum? This is because magenta cannot be emitted as a wavelength of light. Yet magenta exists; you can see it on this color wheel.
Magenta is the complementary color to green, or the color of the afterimage you would see after you stare at a green light. All of the colors of light have complementary colors that exist in the visible spectrum, except for green's complement, magenta. Most of the time your brain averages the wavelengths of light you see in order to come up with a color. For example, if you mix red light and green light, you'll see yellow light. However, if you mix violet light and red light, you see magenta rather than the average wavelength, which would be green. Your brain has come up with a way to bring the ends of the visible spectrum together in a way that makes sense. Pretty cool, don't you think?