Learn in the textbookFundamentals of Color and Image Representation
DefinitionSubtractive color mixing (減法混色) creates colors by overlapping the three primary colors of ink, cyan (C), magenta (M) and yellow (Y), and gets darker the more colors are added.
ExplanationInk absorbs part of the light that hits it, so the more colors are layered, the less light is reflected back to the eye. In theory, layering all three gives black. Real inks alone produce a muddy black, so printers and printing presses add black ink (K) and work with the four colors of CMYK.
AnalogyIt is like stacking sheets of colored cellophane one at a time: what you see through them grows steadily darker.
Memory hookThe K in CMYK is said to come from "Key plate", the printing plate that lays down the black.