Anyone preparing a file for a print shop soon notices that the colors seen on screen do not reach the paper unchanged. The reason is not a printer fault but a completely different way of producing color. A screen emits light; printed paper reflects it, taking part of it away. This article explains the CMYK model and the precautions that avoid the most common surprises.
Subtractive mixing
A white sheet reflects almost all the light that hits it. Each ink acts as a filter absorbing certain wavelengths:
- cyan absorbs red and reflects green and blue;
- magenta absorbs green and reflects red and blue;
- yellow absorbs blue and reflects red and green.
Overlapping inks subtracts more components: magenta and yellow together let only red through, cyan and yellow only green, cyan and magenta only blue. In theory, all three inks at full strength should absorb everything and give black. In practice, because of pigment impurities, you get a dark, muddy brown.
Why black (K) is needed
That is why a fourth ink, black, is added, labelled K (for key, the reference plate the others were aligned to). Black solves three problems:
- it produces truly deep blacks and sharp text;
- it reduces the total amount of ink, and therefore drying time and the risk of set-off;
- it makes grays more stable, because a gray made of three inks shifts hue with the slightest variation in any of them.
Replacing part of the cyan, magenta and yellow with black is called gray component replacement (GCR) or, in its form limited to neutral areas, under color removal (UCR). These are choices the print profile handles during conversion.
Halftoning: how gradations are made
An offset press cannot lay down "half ink" at a point: there is either ink or there isn't. Gradations come from halftoning, that is, dots of variable size arranged on a grid. An area at 30% cyan is covered by cyan dots occupying about 30% of the surface; at normal distance the eye blends dots and paper into an intermediate tone.
The four color grids are rotated at different angles to avoid moiré, the wavy pattern that appears when two regular textures overlap. Digital and inkjet printers often use stochastic screening, with small dots distributed pseudo-randomly.
The total coverage limit
Each paper can absorb only a certain amount of ink. It is measured as the sum of the four channel percentages: an area with C 100, M 100, Y 100, K 100 would have 400% coverage, impossible to print well. The printer usually specifies a limit, which depends on the paper and process: coated papers tolerate more, uncoated paper and newsprint much less.
Exceeding the limit causes smudging, long drying times and sheets sticking together. It is the first thing to ask before preparing a file.
Plain black and rich black
| Type | Indicative values | When to use it |
|---|---|---|
| K-only black | C0 M0 Y0 K100 | text, thin lines, barcodes |
| Rich black | K100 plus cyan, magenta and yellow | large dark areas, backgrounds |
100% black on its own, over a large area, looks dark gray. Adding a base of other inks gives a denser black. But rich black should be avoided on small text: the slightest registration error (plates not perfectly aligned) makes letters show colored halos. The exact rich black values should be agreed with the printer, because they depend on the coverage limit.
The most frequent conversion mistakes
In my work I often receive files "ready for print" that are not. The recurring problems are:
- Out-of-gamut RGB colors: electric greens and blues and fluorescent oranges have no CMYK equivalent and turn dull. Better to find out on screen with soft proofing than on the sheet.
- Black text built from four colors: this happens when an RGB black is converted automatically. The text looks blurred.
- Multiple conversions: going from RGB to CMYK and back several times degrades colors. It is best to keep the RGB original and convert once, with the correct profile.
- Wrong profile: converting with a profile meant for coated paper a job that will be printed on uncoated paper produces oversaturated colors.
- Low-resolution images: a different topic, but often in the same file.
How to prepare a file properly
- Ask the printer for the color profile, coverage limit and guidance on black.
- Work on photographs in RGB and convert them at the end with that profile.
- Set black text to K100 and enable black overprint if requested.
- Check on screen with soft proofing and the out-of-gamut warning.
- Export in a print-oriented format such as PDF/X, which embeds fonts and color information.
In short
- Printing subtracts light: cyan, magenta and yellow absorb red, green and blue respectively.
- Black (K) gives deep blacks, sharp text and lower ink use.
- Gradations come from halftoning, dots of varying size.
- Every paper has a total coverage limit that must be respected.
- K-only text, rich black for backgrounds, a single conversion with the printer's profile.
