CMYK

Why these inks

Printing works by taking light away from white paper. Cyan ink absorbs red light, magenta absorbs green and yellow absorbs blue. Between them they can remove any part of the spectrum. In principle the three together make black. In practice they make a dark brown and soak the paper with ink. A separate black ink gives sharp text and deep shadows, and it is cheaper. The K is usually explained as the key plate, the printing plate that carried the outlines and detail to which the others were aligned.

Dots and screens

A press cannot print lighter or darker ink. It prints solid dots of varying size, a method called halftone. Small dots with plenty of white between them look pale, and large dots look strong. The four colours are printed at different angles so that the dots form a neat rosette pattern instead of clashing. Look at a newspaper photograph through a magnifying glass and the dots are plain to see. Home inkjet printers use a similar idea with much finer, scattered droplets, and often add light cyan and light magenta inks.

Screen to paper

The range of colours that four inks can print is smaller than the range a screen can show. Bright greens, vivid oranges and deep blues often print duller than they looked. Designers convert files from RGB to CMYK before printing and check a proof. The result also depends on the paper: uncoated paper soaks up ink and gives softer colour. When an exact colour matters, such as a logo, printers use a premixed ink called a spot colour instead of building it from dots. Large areas of black are often printed with other inks underneath to deepen them.

Colours on this page

Sources and further reading

• CMYK color model on Wikipedia
• MDN Web Docs: device-cmyk()
• International Color Consortium: CMYK characterization data