A color model in computer terms is a list of numbers which can be used to uniquely identify colors. CMYK, which stands for Cyan Magenta Yellow and Key or Black, is a color model which is used in digital printing. It has now been accepted to refer to the whole printing process. CMYK is sometimes called a subtractive color model since it gives colors by absorbing or ‘subtracting’ certain wavelengths from white light. It achieves very high quality of printing by setting ‘screen angles’ for various colors, which is the angle at which the wavelengths for these colors is absorbed. These angles are varied according to the number of colors needed.
In stark contrast to other color models as discussed below, CMYK is a ‘color on white’ print format.
RGB
This stands for Red Green Black. It is a form of ‘additive’ color model which works by combination of different wavelengths from white light. In this form, black is the default background, and different colors are combined to form new colors. Opposing this, in CMYK, the default background is white, and colors are added to form black. Since this can lead to high ink costs for dark colors, black ink is added to replace the mixing of all primary colors to produce black.
The comparison of the printing quality between CMYK and RGB can not be done, since they both work in very different ways. Their inter-conversion is done using color management systems. This conversion is, however, not exact.
CMYK has only four shades of colors. The way it forms other color shades is called halftoning, and is discussed below.
Halftoning
This is a way for the CMYK color model to produce a continuous range of colors using only the three primary color shades. In this method, a pattern is established which while printing, appears to give a different color. For example, printing a pattern of separated magenta colors with white dots left in between when looked at by the naked eye gives the appearance of light pink. Similarly, patterns can be established for different amounts of different colors and these can give a full range of all colors needed for printing. Halftoning is important since without this only a few basic shades could be produced, obtainable by mixing the three primary shades.
Reason for Presence of Key (Black)
As mentioned above, a mixture of cyan, magenta and yellow in the right proportion would produce black. However, the need to use these inks every time black is required can be both time and cost intensive. Besides this, there are a few other reasons for the presence of black ink –
• All text is usually printed in black. This text requires some level of detail to be useful and to look good. Producing this text by mixing three other colors would blur the text, which can be avoided by directly using black ink.
• A mixture of three inks takes time to dry, making it impractical for large print volumes.
• The three colors, even in a slightly different proportion, can produce light black or even muddy brown. It is better to avoid this occurrence by using key ink.

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