Guide
What Is CMYK?
CMYK is how colour is built out of ink on paper. Four inks, laid down as tiny dots, tricking the eye into seeing a continuous image. Here is how that actually works, and where it stops working.
Four inks, one image #
A full-colour printing press does not mix paint. It lays down four inks — cyan, magenta, yellow and black — one at a time, each as a pattern of tiny dots at a slightly different angle. Stand close enough and you can see the rosette pattern those overlapping dot grids make. Stand at reading distance and your eye blends them into what looks like a continuous photograph.
That is the whole trick. There is no green ink. A green area of a photograph is cyan dots and yellow dots printed near each other, and your eye does the rest.
Why it is called subtractive #
White paper reflects nearly all the light that hits it. Every ink you add takes some of that light away. Cyan ink absorbs red light and reflects the rest, which is why it looks blue-green. Magenta absorbs green. Yellow absorbs blue.
Put all three down at full strength and in theory you absorb everything and get black. This is the opposite of a screen, where you start with a black panel and add red, green and blue light until you reach white. Screens add light; ink subtracts it.
Subtractive. If you can hold onto the idea that ink removes light rather than adding colour, most of the odd behaviour in print colour stops being mysterious — including why the brightest colours on your screen cannot be printed, and why more ink does not mean more vivid.
Why black gets its own plate #
In theory the three chromatic inks make black. In practice they make a muddy dark brown, because real inks are imperfect filters that absorb a bit of the wrong wavelengths. There are three further problems:
- Cost. Black made from three inks uses three times the ink of black made from one.
- Drying. Three layers of wet ink in the same place is a lot of liquid for paper to absorb. It can refuse to dry cleanly and transfer onto the next sheet.
- Sharpness. Black text built from three plates needs all three to align perfectly. A fraction of a millimetre out and the type gets coloured fringes. Printed from a single black plate, it stays crisp.
So a fourth plate carries black on its own. This is also why body text in a print file should be pure K and nothing else.
How percentages work #
Each ink is specified from 0 to 100 percent, and the percentage refers to dot size rather than ink concentration. A 50% cyan tint is not thinner ink — it is full-strength cyan printed as dots that cover half the area. From reading distance, half-covered looks like half-strength.
Add the four percentages together and you get total ink coverage. The mid blue used throughout the converters, at 88 cyan and 44 magenta with no yellow or black, comes to 132%. Presses have a ceiling, often somewhere between 240% and 300% depending on the press and the stock, and going over it causes the drying problems above. Every converter on this site reports that total for exactly this reason.
Where CMYK runs out #
Four process inks cover a smaller range of colours than a screen can display. The gap is most obvious in three places: saturated oranges, vivid greens, and bright blues. Convert a colour like #1E90FF to CMYK and print it, and it will come back noticeably duller. No amount of adjustment fixes this, because the colour is outside what the inks can physically mix.
When a colour genuinely has to be exact — a brand identity, most often — the usual answer is a spot ink: a single pre-mixed ink of that exact colour, printed from its own plate. That is a different system with different economics, and it is the main alternative to process printing.
What this means when you are working #
- Ask your printer what they want before converting anything. Many prefer to receive RGB and convert it themselves.
- Keep your originals in RGB. Converting to CMYK discards information permanently. Convert a copy, at the end.
- Build black text as pure K. Never as a four-ink mix.
- Watch total ink coverage on dark, rich areas.
- Approve colour from a physical proof, not from your monitor.
Questions #
What does CMYK stand for?
Cyan, magenta, yellow and key. Key means the black printing plate, so called because the other three plates were historically aligned — keyed — to it.
Why is black called key rather than black?
Partly to avoid confusion with the B in RGB, and partly because of the historical role of the black plate as the alignment reference for the other three. Both explanations are commonly given and both are reasonable.
Why not just mix black from the other three inks?
You can, and the result is a muddy brown-black. It also costs three times the ink and lays down far too much wet coverage for the paper to handle. A dedicated black plate is cheaper, sharper and dries better, and it lets you print black text with one plate instead of three that must line up perfectly.
Is CMYK used outside printing?
Rarely and only by analogy. It describes ink on paper, so it has almost no application on screens. Design software offers it because designers need to specify print colours, not because screens can display it.
Can CMYK reproduce every colour?
No. Four process inks cover a smaller range than a screen can display, and neither covers everything the eye can see. Saturated oranges, greens and blues are the usual casualties. Spot inks exist partly to fill those gaps.
Do I need to design in CMYK from the start?
Usually not. Most workflows are better served by designing in RGB and converting at the end using the profile your printer specifies. Ask your printer what they want before committing either way.