Why do we perceive a mixture of cyan and magenta as purple under white light?
This is an example of the subtractive color model. White light is a mixture of the three primary colors cyan, magenta and yellow (These colors are used in printer ink. Blue, red and yellow are used in some applications.)
When white light shines on a surface, some wavelengths (colors) may be absorbed. Those that aren't absorbed are reflected and that's what we see. The magenta and cyan components of white light will be reflected by a material that absorbs only yellow light. We see this as a shade of purple, similar to what we would see when red and blue are reflected. In other words, yellow was subtracted from white light, leaving cyan and magenta. Ink jet printers can make a wide variety of colors by using black ink in conjunction with yellow, cyan and magenta inks.