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![]() COLOR MANAGEMENT: Basic Color Theory
By Wasatch, Inc. Staff
The real challenge of printing or displaying color images accurately is this: We are attempting to approximate the colors of the real world using devices or technologies that are not capable of reproducing anywhere near all the colors in the visible spectrum. Furthermore, some of technologies we choose are more capable than others.
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Color Models
RGB is called additive color because it "paints" with light. We use this model because the three wavelengths of the RGB primaries more or less correspond to the signals that are transmitted from the eye the brain. We see in RGB.
RGB is a three-dimensional color space and any color within the space can be described using three numbers. We can represent Cyan, for example, as R = 0, G = 255, B = 255. In Adobe Photoshop and most other application software, levels of the RGB primaries are described in a range from 0 - 255, rather than a percentage from 0 - 100%. When all three primaries are at 255, the screen should be white. When all three primaries are at 0, the screen should be black. When all three primaries are shown at any equal value, the screen should display a neutral gray.
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CMYK and Subtractive Color: Cyan, Magenta and Yellow or CMY is the color model used for most printing devices that print with ink (usually in the form of CMYK). This includes offset presses, inkjet and electrostatic printers. The CMY color space is the theoretical opposite of the RGB color space, but it is also complementary to it.
One of the difficulties with the CMY model is that real life physical inks are less efficient at creating colors than RGB processes, which use light. In addition, some CMY primary colors - notably Cyan - are less efficient than others. These conditions are due to several factors, including variations in raw materials and manufacturing processes. In most cases (but not all), creating colors in a RGB space allows us a wider color gamut than creating colors in CMY.
Obviously, these inefficiencies are a factor when it comes to mixing secondary colors with CM and Y. The "RGB" secondary colors we generate using CMY inks will clearly not correspond to the more ideal color combinations we generate with "real RGB". For example, the Red created by mixing Magenta and Yellow inks will not match the Red primary on a color monitor. However, we can improve our results using color correction and color management methods. Finally, if we mix all three CMY primaries at 100% each, we should get pure black, but in real life, we don't. We get a brownish-gray. This is why Black ink - represented as "K" - has been added to the printing process. We need a "real" black in order to render the deep shadow areas that CMY combinations can't handle. We also need K in order to print "real" black text and linework. Combined with CM and Y, the K channel completes the CMYK color space. Once again, we can describe a color in the four dimensional CMYK color space by using a series of four numbers, each of which is a percentage of a primary. The orange shown in the left column is represented as C= 0, M = 64 Y = 86. These numerical specs are a useful way to describe color, but they are still relative. The color of CMY inks vary dramatically, meaning that a wide range of secondary colors will be produced by mixing the same percentages of different brands, and in some case different batches of the same brand. Go here for a more detailed discussion on inks and media. Although devices that use the RGB color model generally have a wider color gamut than those using CMYK, the additional K channel provides quite a bit of flexibility in controlling tone range on a printed image. Understanding and controlling the behavior of the Black channel is a major advantage in image quality control. In the past couple of years, HIFI and extended gamut inkjet printers have been introduced that add other primary colors to the basic CMYK. This process began with the addition of Orange and Green; the latest models offer other options such as Blue and Red, or even Turquoise or spot colors on the latest fabric printers. So what we really need to handle all of these different inks and output devices is a standard color model where the numeric color description values are universal. There is such a thing...check back soon to read about the L*a*b* color space and ICC Profiles coming soon.
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