Printers that used to work only with cyan, magenta, yellow, and black (CMYK) originated the halftone. Colors that appeared solid were actually fields of tiny dots positioned closely together. The individual halftone dots were a continuous tone, meaning each dot was one unvaried color. All of the cyan dots were the same shade of cyan, and all of the magenta were identical shades of magenta. But, from these three colors and black, a larger spectrum of shades emerged.
When grouped together, the dots appeared to mix and form other shades, especially when viewed at a distance. Cyan, magenta, yellow, and black formed a wide variety of variants based on spacing, dot pattern, and dot size. Using only these four options, printers mixed and matched to make other shades appear.