The "K" in CMYK stands for key since in four-color printing cyan, magenta, and yellow printing plates are carefully keyedor aligned with the key of the black key plate. Some sources suggest that the "K" in CMYK comes from the last letter in "black" and was chosen because B already means blue. However, this explanation, although useful as a mnemonic, is incorrect.
The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks "subtract" brightness from white.
With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary colors; tiny dots of each primary color are printed in a pattern small enough that human beings perceive a solid color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots on the large white paper as lighter and less saturated than the color of pure magenta ink.
Without halftoning, the three primary process colors could be printed only as solid blocks of color, and therefore could produce only seven colors: the three primaries themselves, plus three secondary colors produced by layering two of the primaries: cyan and yellow produce green, cyan and magenta produce a purplish blue, yellow and magenta produce red (these subtractive secondary colors correspond roughly to the additive primary colors) plus layering all three of them resulting in black. With halftoning, a full continuous range of colors can be produced.
To improve print quality and reduce moire patterns (caused when screen dots clash), the screen for each colour is set at a different angle. While the angles depend on how many colors are used and the preference of the press operator, typical CMYK process printing uses any of the following screen angles
The "black" generated by mixing commercially practical cyan, magenta and yellow inks is unsatisfactory, so four-color printing uses black ink in addition to the subtractive primaries. Common reasons for using black ink include:
In traditional preparation of colour separations, a red keyline on the black line art marked the outline of solid or tint color areas. In some cases a black keyline was used when it served as both a color indicator and an outline to be printed in black. Because usually the black plate contained the keyline, the K in CMYK represents the keyline or black plate, also sometimes called the key plate.
Text is typically printed in black and includes fine detail (such as serifs), so to reproduce text or other finely detailed outlines using three inks without slight blurring would require impractically accurateregistration.
A combination of 100% cyan, magenta, and yellow inks soaks the paper with ink, making it slower to dry, and sometimes impractically so. This also can cause the ink to bleed.
Although a combination of 100% cyan, magenta, and yellow inks should, in theory, completely absorb the entire visible spectrum of light and produce a perfect black, practical inks fall short of their ideal characteristics and the result is actually a dark muddy color that does not quite appear black. Adding black ink absorbs more light and yields much better blacks.
Using black ink is less expensive than using the corresponding amounts of colored inks.
When a very dark area is desirable, a colored or gray CMY "bedding" is applied first, then a full black layer is applied on top, making a rich, deep black; this is called rich black. A black made with just CMY inks is sometimes called a composite black or process black.
The amount of black to use to replace amounts of the other ink is variable, and the choice depends on the technology, paper and ink in use. Processes called under color removal, under color addition, and gray component replacement are used to decide on the final mix; different CMYK recipes will be used depending on the printing task
CMYK in Graphic Design
Graphic designers have to deal with the issue of seeing their work on screen in RGB, although their final printed piece will be in CMYK. Digital files should be converted to CMYK before sending to printers, unless otherwise specified. Because of this issue, it is important to use “swatches” when designing if exact color matching is important. Swatches provide a designer and client with a printed example of what a color will look like on paper. A selected swatch colour can then be chosen in Photoshop (or a similar program) to insure the desired results. Even though the on-screen colour won’t exactly match the swatch, you know what your final color will look like. You can also get a “proof” from a printer, which is an example of your printed piece provided before the entire job is run.