CMYK colour model and application

CMYK colour model

CMYK stands for Cyan, Magenta, Yellow and Key (black). It is a subtractive colour model used for printing. Each ink absorbs (subtracts) some of the light falling on white paper, and the colour seen is the light that is reflected back.

• Cyan absorbs red, magenta absorbs green, yellow absorbs blue.
• Each component is a percentage from 0 to 100% of ink coverage. For example (0, 100, 100, 0) is red, and (0,0,0,100) is pure black.
• Conversion from RGB (values 0-1): K = 1 - max(R,G,B), C = (1 - R - K)/(1 - K), M = (1 - G - K)/(1 - K), Y = (1 - B - K)/(1 - K).

The fourth channel K (black) is added because C + M + Y inks together do not make a true black in practice; they give a muddy dark brown.

 C + M = Blue      C + Y = Green      M + Y = Red
 C + M + Y = dark brown  ->  K ink added for true black

Application: colour printing

CMYK is mostly used in commercial and desktop colour printing: magazines, newspapers, books, posters, packaging and inkjet/laser colour printers. Images designed on screen in RGB are converted to CMYK before going to the press.

Reason for use in printing

1. Printing is subtractive. Paper does not emit light; it reflects ambient light, so inks must subtract colours from white, which is exactly what CMY does. RGB (additive light) cannot be used with ink on paper.
2. True black and sharp text. A separate black ink gives deep black and crisp small text without registration problems of three overlapping inks.
3. Saves cost and drying time. Using one black ink in place of equal amounts of C, M and Y saves expensive coloured ink, and less ink makes the paper dry faster and avoids soaking.
4. Better grey and shadow control and a wide range of printable colours with only four plates, one plate for each ink.

A limit is that the CMYK gamut is smaller than RGB, so very bright screen colours (vivid green or blue) look duller in print.