CMYK (short for cyan, magenta, yellow, and key (black), and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. Cyan (saɪæn from Greek κυανός / kyanos, meaning "blue" may be used as the name of any of a number of a range of colors in the blue/green part of Magenta is a purplish red Color evoked by lights with less power in yellowish-green Wavelengths than in blue and red wavelengths ( complements of magenta have Yellow is the Color evoked by light that stimulates both the L and M (long and medium wavelength Cone cells of the Retina about equally Black is the Color of objects that do not emit or Reflect Light in any part of the Visible spectrum; they absorb all such frequencies of A subtractive color model explains the mixing of Paints Dyes Inks and natural colorants to create a range of Colors where each such color A color model is an abstract mathematical model describing the way Colors can be represented as Tuples of numbers typically as three or four values or color components Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the abbreviation. 
The CMYK model works by partially or entirely masking certain colors on the typically white background (that is, absorbing particular wavelengths of light). In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Such a model is called subtractive because inks “subtract” brightness from white. Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light.
In additive color models such as RGB, white is the “additive” combination of all primary colored lights, while black is the absence of light. An additive color model involves Light emitted directly from a source or illuminant of some sort Primary colors are sets of Colors that can be combined to make a useful range ( Gamut) of colors In the CMYK model, it is just the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks. To save money on ink, and to produce deeper black tones, unsaturated and dark colors are produced by substituting black ink for the combination of cyan, magenta and yellow. In Colorimetry and Color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity
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 humans perceive a single color. Halftone is the Reprographic technique that simulates Continuous tone imagery through the use of equally spaced dots of varying size Halftone is the Reprographic technique that simulates Continuous tone imagery through the use of equally spaced dots of varying size Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots and the white paper between the dots 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 six colors: the three primaries themselves, plus three complementary 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 complementary colors correspond roughly to the additive primary colors). With halftoning, a full continuous range of colors can be produced.
To improve print quality and reduce moiré patterns, the screens for individual colors are set at unique angles. While the specific 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 Cyan, Magenta and Yellow primaries is unsatisfactory, and so four-color printing uses black ink in addition to the subtractive primaries. Primary colors are sets of Colors that can be combined to make a useful range ( Gamut) of colors Common reasons for using black ink include:
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. Rich black, in Printing, is an ink mixture of solid black over one or more of the other ( CMYK) colors  A black made with just CMY inks is sometimes called a composite 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. In Four-color printing (or more under color removal (UCR is the process of eliminating amounts of Yellow, Magenta, and Cyan that would have In Four-color printing (or more under color addition ( UCA) is a technique for darkening areas of the printed image by adding colored inks Within the CMY color space any hue angle can be achieved by combining two of the three primaries
CMYK or process color printing is contrasted with spot color printing, in which specific colored inks are used to produce the colors appearing on paper. In Offset printing, a spot color is any color generated by an Ink (pure or mixed that is printed using a single run. Some printing presses are capable of printing with both four-color process inks and additional spot color inks at the same time. High-quality printed materials, such as marketing brochures and books, may include photographs requiring process-color printing, other graphic effects requiring spot colors (such as metallic inks), and finishes such as varnish, which enhances the glossy appearance of the printed piece.
CMYK process printers often have a relatively small color gamut. In color reproduction including Computer graphics and Photography, the gamut, or color gamut (pronounced /ˈgæmət/ is a certain complete Processes such as Pantone's proprietary six-color (CMYKOG) Hexachrome can considerably expand the gamut. Pantone Inc is a corporation headquartered in Carlstadt, New Jersey, USA Hexachrome is Pantone 's six-color Color printing process In addition to custom CMYK inks Hexachrome adds orange and Green inks to Additionally, light, saturated colors often cannot be created with CMYK, and light colors in general can make visible the halftone pattern. Using a CcMmYK process, with the addition of light cyan and magenta inks to CMYK, can solve these problems, and such a process is used by many inkjet printers, including desktop models. CcMmYK is a six color printing process used in some Inkjet printers optimized for photo printing Inkjet printers operate by propelling variably-sized droplets of liquid or molten material ( Ink) onto almost any sized page 
Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are so different. A laser or ink-jet printer prints in dots per inch (dpi) which is very different from a computer screen, which displays graphics in pixels per inch (ppi). Dots per inch ( DPI) is a measure of spatial Printing or Video dot density in particular the number of individual dots that can be placed within the span A computer screen mixes shades of red, green, and blue to create color pictures. A CMYK printer must compete with the many shades of RGB with only one shade of each of cyan, magenta and yellow, which it will mix using dithering, halftoning or some other optical technique; this dithering produces a lower level of detail than the printer's dpi suggests. Dither is an intentionally applied form of Noise, used to randomize Quantization error, thereby preventing large-scale patterns such as contouring that are more objectionable Halftone is the Reprographic technique that simulates Continuous tone imagery through the use of equally spaced dots of varying size
Since RGB and CMYK spaces are both device-dependent spaces, there is no simple or general conversion formula that converts between them. Conversions are generally done through color management systems, using color profiles that describe the spaces being converted. In digital imaging systems color management is the controlled conversion between the Color representations of various devices such as Image scanners Digital In Color management, an ICC profile is a set of data that characterizes a color input or output device or a Color space, according to standards promulgated by the Nevertheless, the conversions cannot be exact, since these spaces have very different gamuts. In color reproduction including Computer graphics and Photography, the gamut, or color gamut (pronounced /ˈgæmət/ is a certain complete
The problem of computing a colorimetric estimate of the color that results from printing various combinations of ink has been addressed by many scientists.  A general method that has emerged for the case of halftone printing is to treat each tiny overlap of color dots as one of 8 (combinations of CMY) or of 16 (combinations of CMYK) colors, which in this context are known as Neugebauer primaries. Hans E J Neugebauer was a German -born Physicist and imaging scientist who later lived in the United States and Canada. The resultant color would be an area-weighted colorimetric combination of these primary colors, except that the Yule–Nielsen effect ("dot gain") of scattered light between and within the areas complicates the physics and the analysis; empirical formulas for such analysis have been developed, in terms of detailed dye combination absorption spectra and empirical parameters. Dot gain is a phenomenon in Printing and Graphic arts whereby printed dots are perceived and actually printed bigger than intended