Printing builds color by subtracting light, not adding it.
Screens add light (red, green, blue), but ink works the opposite way: each color absorbs part of the light hitting the page. Printers use cyan, magenta, and yellow as their primaries because each subtracts one of the eye's three channels. Magenta ink absorbs green, so the red and blue light bouncing back is exactly what your brain reads as magenta.
