CMYK is the four-ink process used for most commercial colour printing: cyan, magenta, yellow and key black. Unlike a screen, paper does not emit light. It reflects light through layers of ink, so colour is created by subtracting parts of the visible spectrum from the white of the paper.
The letters are not a promise that every visible colour can be printed. They describe a process with a specific, limited gamut. The stock, press, inks, screening method and output profile determine the actual result.
Four separations, one printed image
An image is converted into four separations: one for C, M, Y and K. Each separation controls how much of that ink is laid down at every point on the sheet. At normal viewing distance, the tiny dots overlap and blend into continuous-looking tone and colour.
Black is included because combining CMY inks alone makes neutral shadows inefficient and can look muddy. The K plate adds detail, contrast and economical shadow density. It is also why small black text is usually set as 100K rather than built from four colours; read more in rich black vs plain black.
CMYK is subtractive; RGB is additive
On a display, adding red, green and blue light moves toward white. In print, adding ink absorbs more light and generally moves toward a darker result. That difference explains why a vivid RGB colour can be out of gamut for CMYK, and why paper colour can shift even a carefully prepared design.
The goal of colour management is not to make every screen colour printable. It is to translate the original predictably for a known printing condition. A coated-sheet offset press, an uncoated stock, a digital press and newsprint do not share the same CMYK gamut.
Halftones make tones possible
A press does not vary the thickness of cyan ink smoothly from pale to dark in the way a screen varies light. Instead, it uses patterns of very small dots. Smaller or more widely spaced dots make lighter apparent tones; larger dots make darker ones. The dot patterns for the four inks are screened at angles that reduce visible interference.
See halftones and line screens for how screening relates to image resolution and print texture.
Total ink coverage protects the sheet
Every CMYK object has a total area coverage (TAC): the sum of its four ink percentages. A colour of 60C / 40M / 40Y / 100K has 240% TAC. If too much ink is applied for the stock and process, it can cause slow drying, set-off, muddy shadows or loss of detail.
There is no universal maximum. Use the ICC profile or output specification supplied for the job. A figure suitable for coated offset paper may be too high for uncoated paper or newsprint.
A reliable CMYK workflow
- Ask the printer or production team for the intended printing condition, PDF standard and output profile.
- Keep an editable master in a wide enough colour space while you work, unless the delivery specification says otherwise.
- Use soft proofing to preview the intended CMYK condition when it is available.
- Convert only when the production workflow requires it, using the agreed profile.
- Preflight the final PDF for colour spaces, spot colours, overprint and TAC before release.