CMYK color model

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Cian, magenta, yellow and key (black).

Subtractive model cian, magenta and yellow.

The CMYK model (abbreviations of Cyan, Magenta, Yellow and Blac K) is a subtractive color model used in color printing. It is the modern and more precise version of the old traditional coloring model (RYB), which is still used in painting and plastic arts. It allows to represent a wider range of colors than the latter, and has a better adaptation to industrial environments.

This model is based on the mixture of pigments of the following colors to create others:

C = Cyan (Cian).
M = Magenta (Magenta).
Y = Andellow (Amarillo).
K = BlacK (Black).

Black is called key, instead of using the letter B, as it is a short name for the term key plate used in printing. This master plate printed the artistic detail of an image, usually in black ink. The use of the letter K also helped to avoid confusion with the letter B used in the RGB acronym.

Mixing of ideal CMY colors is subtractive (since mixing of cyan, magenta, and yellow on a white background results in the color black). The CMYK model is based on the absorption of light. The color of an object corresponds to the part of the light that falls on it and is not absorbed by the object.

Cyan is the opposite of red, which means it acts as a filter that absorbs red (-R +G +B). Magenta is the opposite of green (+R -G +B) and yellow is the opposite of blue (+R +G -B).

Three examples of color use with CMYK separation and using halftone. From left to right: the component drank, the magenta, the yellow, the black, the combined halftone model and how the human eye would perceive them at sufficient distance.

Using Black Ink

For various reasons, the black generated by mixing the subtractive primaries is not ideal, and therefore four-color printing uses black in addition to the subtractive primaries yellow, magenta, and cyan. Among these reasons are:

A mixture of yellow, cian and magenta pigments rarely produces pure black because it is almost impossible to create enough amount of pure pigments.
Mixing the three inks only to form the black can moisten the paper if a dry toner is not used, which implies a problem in printing in large strips, in which the paper should dry quickly enough to prevent the marking of the next sheet. In addition, the low quality paper, such as the paper used for newspapers, can be broken if it is too humid.
The text is often printed in black and includes fine details if the typography is with serif. In order to reproduce the text using three inks without fading or slightly blurring the typographical symbol, an extremely accurate record would be required. This way of generating the black color is not possible, in practice, if you want a faithful reproduction in the density and outline of the typography (by having to align the three images with too much accuracy).
From an economic point of view, the use of a black ink unit, instead of three color ink units, can mean a great savings, especially because black ink is usually much more economical than any color ink.

The amount of black to be used, to replace the amounts of the other inks, is variable and the choice depends on the technology, the type of paper and the kind of ink used. Processes such as undercolor removal, undercolor addition and gray component replacement are used to decide the final mix, whereby different CMYK recipes will be used depending on the print task. When black is mixed with other colors, it results in a blacker black called "rich black" or "super-black". The percentage of each ink to achieve this rich black will depend on the printing method, the type of paper it is printed on, and the printer's specifications. The "log black" it is the sum of 100% of the four CMYK inks and is called "registration black" since it is used for registration marks (registration crosses and crop marks). The "log black" It should not be used during the design process of the printed product, since the paper cannot support the sum of 100% of the four inks.

Inks

Chromatic circle produced from the primary colors of the CMY/CMYK model.

Typical CMYK coloring pigments are copper phthalocyanine cyan (pigment blue 15:4), quinacridone magenta (pigment red 122), diarylated yellow (yellow 12), and carbon black.

Comparison with the RGB model

Using four-color printing produces a good result with higher contrast. However, the color seen on a computer monitor is often different than the color of the same object in a print, as CMYK and RGB models have different color gamuts. For example, pure blue (In 24 and 32 bits= RGB=0,0,255) is impossible to reproduce in CMYK, however, very similar tones can be achieved.

Computer monitors, and other displays, use the RGB model, which represents the color of an object as an additive mixture of red, green, and blue light (the sum of which is white light). In printed materials, this combination of light cannot be reproduced directly, so computer-generated images, when using an editing, vector drawing, or photo retouching program, must be converted to their equivalent in the CMYK model that it is suitable when using a device that uses inks, such as a printer, or an offset machine.

Conversions

Printed reference photo using a cuatricromy process.

It is interesting to note that the conversions mentioned here are of the nominal type. They will produce a reversible conversion between RGB and a subset of CMYK; that is to say, you can choose a color from the RGB palette and convert it to certain CMYK colors, and from these CMYK colors obtain the original colors that correspond to them again in the conversion to RGB. However, the reverse conversion (from CMYK colors to RGB) is generally not reversible; that is to say that a color given in CMYK and that is converted to RGB, will not result in the original CMYK color when it is converted back to the CMYK palette.

In addition, CMYK colors can be printed in very different tones than they appear on a monitor. There is no "good" conversion rule between RGB and CMYK, because neither model represents an absolute color space.

Conversion between RGB and CMYK

Comparative image showing the differences in color between the RGB (left) model and the CMYK (right).

To convert between RGB and CMYK, an intermediate CMY value is used. The color values are represented as a vector, each one of them being able to vary between 0.0 (non-existent color) and 1.0 (fully saturated color):

${displaystyle t_{CMYK}!,}$	${displaystyle ={C,M,Y,K}!,}$ is the CMYK quadruple in ${displaystyle left[0,1right]^{4}}$ ,
${displaystyle t_{CMY}!,}$	${displaystyle ={C,M,Y}!,}$ is the triple CMY in ${displaystyle left[0,1right]^{3}!,}$ ,
${displaystyle t_{RGB}!,}$	${displaystyle ={R,G,B}!,}$ is the triple RGB in ${displaystyle left[0,1right]^{3}!,}$ .

CMYK to RGB conversion

To achieve the conversion, you first go from CMYK to CMY, and then to RGB.

${displaystyle t_{CMYK}!,}$	${displaystyle ={C,M,Y,K},}$
${displaystyle t_{CMY}!,}$	${displaystyle ={C',M',Y'}!,}$
	${displaystyle ={C(1-K)+K,M(1-K)+K,Y(1-K)+K},}$
${displaystyle t_{RGB}!,}$	${displaystyle ={R,G,B}!,}$
	${displaystyle ={1-C',1-M',1-Y'},}$
${displaystyle t_{RGB}!,}$	${displaystyle ={1-(C(1-K)+K),1-(M(1-K)+K),1-(Y(1-K)+K)}!,}$
	${displaystyle ={(1-C)(1-K),(1-M)(1-K),(1-Y)(1-K)},}$

RGB to CMYK mapping

As you can see, the top image has been obtained by superimposing the four layers of the bottom. Note that each of the lower images corresponds to a basic color of the CMYK model.

A given RGB color can be mapped to one of many possible semi-equivalent CMYK colors. The best option is one that makes use of K as much as possible, and remaining proportions of CMY as little as possible. For example, #808080 (gray, exactly halfway between black and white) will be mapped to (0,0,0,0.5) and not (0.5,0.5,0.5,0).

Converting RGB → CMY, with the same color vectors:

{displaystyle t_{RGB}={R,G,B},}

converting to CMY

{displaystyle t_{CMY}={C',M',Y'}={1-R,1-G,1-B},}

and then to CMYK:

Yeah.

{displaystyle min{C',M',Y'}=1,}

then.

{displaystyle t_{CMYK}={0,0,0,1},}

otherwise

{displaystyle K=min{C',M',Y'},}

{displaystyle t_{CMYK}=left{{frac {C'-K}{1-K}},{frac {M'-K}{1-K}},{frac {Y'-K}{1-K}},Kright},}

Its use in graphic arts

Its widespread use occurs in the context of graphic arts. Offset printers generally print in these four colors plus special spot inks, if applicable (commonly called Pantone colors). For this reason, before sending any work to the printer, the colors of the document must be converted to CMYK, so that the printing colors are as correct as possible.

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