Printing colour built from dots of cyan, magenta, yellow and black ink, rather than a separately mixed ink for each colour.
Halftone dots of the four process inks overlap and sit side by side; the eye blends them. The achievable gamut depends on the inks, the paper and the press, which is why CMYK values need a profile to mean a specific colour.
CMYK is not one colour space. The same CMYK numbers print differently on coated and uncoated paper.
Most magazines and brochures are printed in process colour.
It explains why some screen colours, especially bright blues, greens and oranges, cannot be printed as they appear.
A printing condition is characterised by printing a standard chart of CMYK combinations on the intended paper and measuring every patch in CIELAB under D50. The published data sets, such as FOGRA51 for premium coated offset, are the result, and press profiles are built from them. On press, the solids and overprints are checked against those aims with a spectrophotometer, and tone value increase (dot gain) is checked on tint patches. The premium coated data gives a paper white of L* 95 and a black solid of L* 16, which bounds the tonal range of anything printed that way.
| Printing condition | Paper white L* | Largest printable chroma | ΔE00 lost |
|---|---|---|---|
| Offset, premium coated paper (PSO Coated v3) (FOGRA51) | 95.0 | 81.4 | 4.4 |
| Offset, wood-free uncoated paper (PSO Uncoated v3) (FOGRA52) | 93.5 | 50.7 | 12.0 |
| Coldset offset on standard newsprint (IFRA26) | 85.2 | 45.4 | 13.5 |
Scarlet #ff2400 has a CIELAB chroma of 104.3. Coated paper can print 81.4 of it, uncoated paper 50.7 and newsprint 45.4, so the same file loses 4.4, 12.0 and 13.5 ΔE00 respectively. The inks are cyan, magenta, yellow and black in each case. The paper decides how much colour they can build.
How this was calculated: The sRGB colour is adapted to D50 and tested against each condition's published solid patches with Colourwise's cusp-triangle gamut model, an approximation of what a full profile would report.
A design with a deep blue background is printed on uncoated stationery as well as a coated brochure.
The same CMYK build is darker, greyer and weaker on the uncoated sheet, because the paper absorbs ink and the available chroma drops sharply. Each stock needs its own conversion from the original colour.
Small text or fine lines are set in a colour made from three or four inks.
The plates never align perfectly, and the slightest misregister gives the type coloured fringes. Fine detail is safest in a single ink.