CMYK: printing ink

CMYK describes a color as four printing inks, Cyan, Magenta, Yellow and Black, each from 0 to 100 percent. It is the language of the print shop. Here it is both a reading and a way in: read off the numbers to give to a printer, or type in numbers a printer has given you.

What it does

With CMYK chosen under Color space, each card shows four sliders, C, M, Y and K. Each letter sits on a disc of its ink. Each track runs from none of that ink to all of it, with the other three held, so you can see how much cyan would cool the color before you add it. Type exact values into the number boxes and press Enter.

The numbers come from the simple textbook conversion, the one used when no press profile is involved. Black, K, is how far the color’s brightest light falls short of full. Cyan, magenta and yellow then carry what color remains. Three things follow:

• In a reading, at least one of C, M and Y is always 0.
• Every gray, black included, is K alone. Black is K 100.
• #3366CC, a mid blue, reads C 75, M 50, Y 0, K 20.

The inks you set stay as you set them. Push the third ink up, to C 75, M 50, Y 40, K 20, and the sliders keep those four numbers, whether you dragged there, stepped with the arrow keys or typed. The color is read off afresh only when it changes somewhere else: the hex field, the wheel, a harmony, a swap, an undo. Then the gray that the three inks share is handed to K, and the same color reads C 58, M 16, Y 0, K 52. A gray you reach with the sliders keeps the hue it had, so its marker on the wheel does not swing around to red.

These numbers describe a screen color under a simple rule. They are not what a particular press, paper and ink will print. For a job that matters, your printer will convert with a profile made for their machine.

Why it works this way

Designers are handed CMYK numbers all the time: a brand color written as “75/50/0/20”, a swatch from a print job. The app lets you type them in and read them out, using the simplest rule that connects ink numbers to screen color. A true press conversion needs a color profile for a specific printer and paper, and a design tool that pretended to have one would be misleading.

The app keeps the color itself in one place and reads CMYK off it, rather than storing four ink values. Four inks describe a three-dimensional thing, so many ink mixtures make the same color. Storing them would mean inventing the fourth number whenever a color arrived from somewhere else. Reading them off means the color never drifts when you switch models.

The inks you have just set are the exception. Four inks can make one color in many ways, and working the reading out again after every step of a drag took a little of the ink you were dragging and handed it to K: dragging Y from 0 to 40 on #3366CC ended on a dark green, #32663D, where typing 40 gives #33667A. So a card remembers the four numbers you set for as long as the color is still the one they made. It is a note of what you did, not a second copy of the color, and nothing is ever made up for a color that came from elsewhere.

Carrying gray on the black plate has real advantages on press. A neutral printed with more black and less of the three colors uses less ink, dries faster and is less likely to pick up a color cast if the plates shift slightly out of line. Printers call this gray component replacement and usually apply it in part; this conversion takes it all the way.

A notan is cut, not printed, so why include CMYK at all? Because notans are photographed, printed and published. And because CMYK is the industrial descendant of RYB: seeing the two side by side shows why printers gave up red and blue.

The metaphor

The picture behind CMYK is four sheets of colored glass laid over white paper. Each ink is a filter that takes some light away: cyan takes red, magenta takes green, yellow takes blue. Black takes everything. That is why there is no white slider. In print, white is simply paper with no ink on it, and the lightest color you can print is the paper itself.

The name K stands for key, the printer’s word for the black plate, which carries the drawing and the deepest darks; in the usual explanation, the color plates were lined up to it. You will also hear that K was chosen to avoid confusion with B for blue; both explanations are told.

The metaphor breaks on press. Real ink is not laid down as smooth sheets of glass but as dots, a halftone. Each ink is printed as a grid of dots whose size varies with the tone, and the four grids are turned to different angles, the darkest traditionally at 45 degrees, where the eye least notices rows. Up close the dots form small rosettes. Some overlap, some sit side by side, and the eye mixes them at a distance. Dots also spread as ink soaks into paper, so a 50 percent tint prints darker than 50. And the range of colors a press can print is smaller than a screen can show. Some screen blues and greens have no CMYK equivalent at all; this app will not warn you.

A history

In art

CMYK is a printer’s tool, but artists have made its dots their subject. In Look Mickey (1961), Roy Lichtenstein first painted the Ben-Day dots of cheap comic printing, a technique for shading with dots invented by the printer Benjamin Day in 1879. In the 1960s in Germany, Sigmar Polke painted the halftone dots of newspaper photographs one at a time, using the rubber end of a pencil. Both made visible what print is designed to hide: a picture is an agreement between dots and a distant eye.

Georges Seurat had worked on the same agreement from the other side. His paintings of the 1880s placed small touches of color side by side for the eye to mix, following the color science of Chevreul and others. Pointillism is not CMYK, but it is the same bet.

In printing

Le Blon printed with red, yellow and blue plates in the 1720s and sometimes added a black. Gautier d’Agoty, who had worked for him, made the black plate part of his own method. In 1868 Louis Ducos du Hauron patented a way to photograph a scene three times through colored filters and print the three records in complementary pigments: the principle of the color separation.

What made cheap color printing possible was the halftone. William Fox Talbot proposed photographic “screens” in an 1852 patent. Frederic Ives patented a successful commercial method in 1881, and Georg Meisenbach his “autotype” in 1882. In the 1890s Max and Louis Levy of Philadelphia made precise cross-lined glass screens. Four-color process printing came into use for newspaper illustrations and comic strips in the 1890s. Over the twentieth century the inks moved toward the cyan and magenta used today.

The other tradition never went away. In 1963 Lawrence Herbert launched the Pantone Matching System, a book of numbered colors, each with a recipe from a small set of base inks. Printers still use spot inks when a color must be exact, and process inks when a picture must be full of color.

In technology

Separations were once made with cameras and filters by skilled craftsmen. From the second half of the twentieth century electronic drum scanners took over, reading an original through red, green and blue filters and computing the four plates. In the mid-1980s desktop publishing put page layout on personal computers. Photoshop 2.0 in 1991 added a CMYK mode, bringing separations to the desktop. In 1993 Adobe, Agfa, Apple, Kodak, Microsoft, Silicon Graphics, Sun and Taligent founded the International Color Consortium, whose profiles let a CMYK number mean a particular color on a particular press. Your desk inkjet is a CMYK press in miniature, and many add lighter inks to hide the dots.

In professional software

In print production CMYK is called process color. What transfers from Studio Notan is that CMYK numbers are a translation. Professional programs translate through a profile for a particular press and paper, so the same color will usually read with different numbers there.

• Adobe Illustrator. Set print work to ‘File › Document Color Mode › CMYK Color’. ‘Window › Separations Preview’ shows each ink plate on its own, and ‘View › Overprint Preview’, ⌥⇧⌘Y (Alt+Shift+Ctrl+Y), shows inks printed over one another. A swatch is either a process color, built from the four inks, or a spot color with an ink of its own.
• Adobe InDesign. ‘Window › Output › Separations Preview’, ⇧F6 (Shift+F6), turns the plates on and off one at a time. Switch its view to ‘Ink Limit’ to highlight any area where the four inks add up to more than the total your printer allows.
• Adobe Photoshop. ‘Image › Mode › CMYK Color’ converts a picture with the CMYK profile set in ‘Edit › Color Settings’. Preview first with ‘View › Proof Colors’, ⌘Y (Ctrl+Y); ‘View › Gamut Warning’, ⇧⌘Y (Shift+Ctrl+Y), marks in gray the colors the press cannot print. In the ‘Color Picker’ a warning triangle flags an unprintable color; click it for the nearest printable one.
• Autodesk AutoCAD. The ‘Color Books’ tab of the ‘Select Color’ dialog lists Pantone colors by name, so a drawing can name the spot ink a sign or panel should be printed in. On screen each one is shown by an RGB stand-in.

What carries over. Ask the printer before you convert: which profile, what total ink limit and how they want black built. Large areas of K 100 alone can print as a dark gray, so printers often ask for a rich black, with some cyan, magenta and yellow beneath it; small text should stay K alone, so that slight misregistration cannot fringe it with color. And do not paste this app’s CMYK numbers into a print file expecting a match: they come from the simple formula, not a press profile.

Try this

1. Type #3366CC into the Front card’s hex field and switch to CMYK: C 75, M 50, Y 0, K 20. Those are the numbers a printer would expect for that blue.
2. Now drag Y up to 40. The sliders stay at 75, 50, 40, 20. Type the code the Front card now shows, #33667A, into the Back card’s hex field: there the same color reads C 58, M 16, Y 0, K 52. Two sets of inks, one color.
3. Drag C, M and Y down to 0 and move K alone. You get a ramp of pure grays: the key plate by itself.
4. Set every ink to 0 (white), then raise M and Y to 100. You get red, #FF0000. A printer makes red from two inks; in RYB it is a single paint.

Going further

The painter’s ancestor of these inks is in RYB: mixing paint. The lights that separations are photographed through are in RGB: light. For how the four models share one color, see Color space.

Books: Bamber Gascoigne, How to Identify Prints; Sarah Lowengard, The Creation of Color in Eighteenth-Century Europe.