|Red, Green, and Blue are the "additive colors " -
combine red, green and blue light, and you get white light. Cyan,
Magenta and Yellow are "subtractive colors" - if you print cyan, magenta
and yellow inks on paper, they ought to absorb all the light shown on
them. Your eye receives no reflected light from the paper, and perceives
black... in theory.
In practice, printing inks contain impurities that prevent
them from absorbing light perfectly. They do a pretty good job with
light colors, but when you add them all together, they produce a murky
brown rather than black. In order to get decent dark colors, black ink
is added in increasing proportions, as the color gets darker and darker.
An image that is in RGB mode is optimized for display on a computer monitor. In order to reproduce that very same image using ink on paper, it must be converted to the "CMYK" color mode.
RGB gamutThe word "gamut" is used to describe a range of reproducible color with a given set of tools. There are some colors which a computer monitor can display which are impossible to print using the standard "SWOP CMYK" inks in use across the United States (SWOP: standard web offset press). Most notably, certain vibrant deep blues and rich reds are "outside the gamut" of SWOP CMYK.
|There are also colors which are reproducible with CMYK inks which are impossible to represent on a computer screen. Pure Cyan is particularly problematic.|
|In order to print properly, any image files that
you supply for CMYK printing must be in CMYK mode [note that disc art is
rarely CMYK]. RGB files will look good on screen, and they will even
look good when printed on many of the desktop color printers on the
market today. However, they will not separate properly when made into
film, and the resulting full printing job will not look the way you
expect it to look.
Inexperienced graphic designers, unfamiliar with the
limitations of the SWOP CMYK gamut, supply us with a steady stream of
RGB files, which we relentlessly convert to CMYK mode before sending for
film output. Much of the time, the color change that occurs is slight.
Every once in a while, though, we get artwork whose effectiveness is
severely compromised when the color range is compressed during the
transition to CMYK mode. It is often a challenging task to explain to
the designer why there is absolutely no way to get that blue using CMYK,
no matter how much we want to.|
Recommended Workflow for designing for CMYK printingHere are a few application specific tips.
Photoshop: Your scanner almost certainly generates RGB information. Don't worry, that's how it's supposed to work. In fact, you should leave your color files in RGB mode up until you need to print separations, or until you need to know CMYK ink values, so you can match colors in another program. While you're working, check how your files are going to look by turning on the "CMYK preview" mode. Don't make repeated changes between RGB and CMYK mode, using the mode menu. Every time you switch, a little clarity is lost. One switch is no problem; 20 switches makes a difference.
You may ask, why not simply switch to CMYK mode as soon as possible? 1) RGB files are 25% smaller, and are therefore 25% faster to work with and easier to store. 2) The SWOP CMYK gamut is pretty small. If you ever want to reproduce those files for a different medium (such as the web), you'll have thrown away some potentially useful information 3) Some filters only work in RGB mode.
Illustrator: Stick to CMYK and Greyscale color models. If you use Pantone Coated colors, make sure that 1) you convert them into CMYK mode or 2) you leave them as spot colors because you want to print spot color inks (and you're willing to pay all the associated upcharges). Stay away from RGB.
Quark Xpress: Familiarize yourself with the "Edit Colors" dialog box. Use only CMYK model and Pantone Coated model ink definitions. Make sure that you are in control of which colors should separate into CMYK and which should print as spot colors. Be aware that Quark's ability to represent color accurately is, shall we say, less than ideal.
In color reproduction, including computer graphics and photography, the gamut, or color gamut is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. Another sense, less frequently used but not less correct, refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process.
In color theory, the gamut of a device or process is that portion of the color space that can be represented, or reproduced. Generally, the color gamut is specified in the hue–saturation plane, as a system can usually produce colors over a wide intensity range within its color gamut; for a subtractive color system (such as used in printing), the range of intensity available in the system is for the most part meaningless without considering system-specific properties (such as the illumination of the ink).
When certain colors cannot be expressed within a particular color model, those colors are said to be out of gamut. For example, while pure red can be expressed in the RGB color space, it cannot be expressed in the CMYK color space; pure red is out of gamut in the CMYK color space.
A device that is able to reproduce the entire visible color space is an unrealized goal within the engineering of color displays and printing processes. While modern techniques allow increasingly good approximations, the complexity of these systems often makes them impractical.
While processing a digital image, the most convenient color model used is the RGB model. Printing the image requires transforming the image from the original RGB color space to the printer's CMYK color space. During this process, the colors from the RGB which are out of gamut must be somehow converted to approximate values within the CMYK space gamut. Simply trimming only the colors which are out of gamut to the closest colors in the destination space would burn the image. There are several algorithms approximating this transformation, but none of them can be truly perfect, since those colors are simply out of the target device's capabilities. This is why identifying the colors in an image which are out of gamut in the target color space as soon as possible during processing is critical for the quality of the final product.
Representation of gamutsGamuts are commonly represented as areas in the CIE 1931 chromaticity diagram as shown at right, with the curved edge representing the monochromatic (single-wavelength) or spectral colors.