"A beam of white light is made up of all the colours in the spectrum. The range extends from red through to violet, with orange, yellow, green and blue in between."
It's true, the color green, in the light spectrum has no complimentary color (the color opposite to it) within the spectrum itself, every other color does. It, for whatever reason doesn't need it but our brains actually do and we can see this because of afterimages like the one below stare at the four dots in the center for about 30 seconds then look into the white space to the right.
In the case of magenta, when we see magenta (being complementary color to green as mentioned before) since this color doesn't exist in the spectrum, our brains make up the difference between the two ends of the spectrum (red and violet) creating magenta.
It's all in our heads, no colors actually "exist", only perceived. It's a very interesting article, but pretty difficult to interpret unless you specialize in that sort of science (which by my summing up you could probably tell that I am not). I did my best to put the article in laymen's terms.