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Neither your monitor, scanner nor any other device is capable of reproducing the entire spectrum of colors that are distinguishable by the human eye. Each device operates within a specific color space that can produce a certain range, or gamut, of colors.
A computer monitor (and your TV) display RGB color – the primary colors of Red, Green and Blue. The total number of colors that can be reproduce in RGB is known as the RGB color space. RGB is an additive color where light is emitted directly from an illuminant and overlaps to create colors.
Commercial printers can not print with light, obviously, so they can not print in RGB color. All of the colors of a color photograph are simulated on a printing press by mixing four ink colors, known as process ink colors:
Cyan, Yellow, Magenta and Black.
The set of all of the possible colors that can be printed in process color is known as the CMYK color space. CMYK are subtractive colors, as colors are caused by subtracting (that is, absorbing) some wavelengths of light and reflecting the others.
Although a monitor is not capable of displaying every color in the color spectrum, it does have a very wide color space. Many colors on an computer monitor, especially very bright colors, can not be reproduced accurately on a printing press using the CMYK process color inks. This is because the CMYK color space is much smaller than the RGB color space. The RGB colors that can't be achieved are said to be“out of gamut” of the CMKY color space available in commercial printing with process color.
All professional design programs have both RGB and CMYK color space options. If you are creating a design that will only be used for screen display, such as a website, RGB color is the proper color space to setup your document in. BUT, if the document will be printed on a printing press in process color, you want to be certain that your document in the CMYK color space.
If your document is rich with bright, vivid color, and you change the document to the CMYK color mode, you may immediately notice that you lost a lot of the vividness of color. Yes, it may not look as good on your monitor, but at least you're seeing the true representation of what you can expect on the actual print job.
Every good rule has an exception. If you are designing only for spot color output, such as a 2-color project with two Pantone Solid Colors, you could make the document in RGB color. The only reason you might want to do that is if the ink colors are way out of gamut of the CMYK color space, and you want to proof a PDF that more accurately represents the final output of the printing project.
Notice the round illustration above: the RGB color space (yellow line) is very large, and the CMYK color space (blue line) is quite small. The color space of Pantone Solid Colors (red line) provide a much larger number of possible ink colors to choose from.
The only problem in using the Pantone Spot Colors is expense, as they are generally only used when printing a 1 or 2-color document. When printing a full color document you are already printing in four colors. Adding one or two Pantone Solid Colors makes the project a 5 or 6-color design, driving up the cost. A common reason that solid PMS colors are added to an full-color design is for exact color-matching a corporate logo when the color shift is significantly different when converted to CMYK color.
» » Read more about Pantone Ink Colors.