A way that I have had the idea of this explained is this. There are two ways to experience most colours.
Take yellow. You could experience yellow by seeing pure yellow light which is at the yellow wavelength. The other way to experience yellow is by seeing a mixture of red and green wavelength light.
Your eye can't tell the difference between these two types of yellow. The yellow as reflected off a banana is "real" yellow. The yellow you see coming out of your computer monitor is actually just a mix of red and green.
Magenta is interesting because the only way to experience it is by a combinations blue and red. There is no "real" magenta.
This video explains this concept very well: