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CMYK
The acronym CMYK stands for cyan, magenta, yellow, and a key color which is almost always black. The computer CMYK mode is used only for images that are going to be reproduced on a commercial press or on one of the specialty printers that requires CMYK, such as the Iris printer.
CMYK colors are called process colors. Because the press uses these four inks to create all the colors an image needs, printing with CMYK is called four-color printing, or process printing. People unfamiliar with the terminology call it full-color printing simply because the result does look like (and is) full color.
The CMYK color model is based on what happens with light and objects out in the world, rather than in a monitor. A light source such as the sun or a lightbulb sends white light down to objects around us; certain colors of the spectrum are absorbed by the objects and certain colors are reflected back to our eyes. For instance, when light hits a red apple, the apple absorbs (subtracts) all the colors of the light except the red, and the red is reflected into our eyes. In physics, this is called a subtractive color model. One hundred percent of cyan, magenta, and yellow creates (in theory) black. (Remember, in RGB one hundred percent of red, green, and blue creates white.)
Similar to RGB mode, there is a channel in image editing programs for each of the four transparent colors in CMYK mode. The channels show the amounts of each process color that will be printed. These are also called the separations for the image. The combination of all four channels is called the composite image.

Cyan

Magenta

Yellow

Black

Composite

The cyan, magenta, yellow and black channels of a scan combine to create the CMYK images. These are the same "separations" that will print during the commercial printing process.


RGB
The acronym RGB stands for red, green, and blue. This RGB is the system monitors use to create color, using light. Monitors have three "guns" inside that "shoot" red, green, and blue light to every pixel on the screen. The computer blends these three light beams together in varying proportions to create the other colors you see. One hundred percent of all three colors produces white, which is why RGB is called an additive color model. In RGB, red light mixed with green light creates yellow light.
What, red and green makes yellow? That's not what you expect when you mix paint colors, is it? That's because when you mix colors in the world, the light comes from a source like the sun or a lamp, bounces off the paint in the bucket, and reflects the color back into your eyes. The physics of color in a monitor is completely different: the light does not bounce off of any physical object-it is projected straight into your eyes. Television, video monitors, and lighting work the same way, using red, green, and blue light.

Scanners use RGB to capture color images. A scanner captures the varying levels of all the red, green, and blue data in an image. Each set of color information is called a channel. When the three channels of color are combined, the result is the full-color image.

Each of these RGB channels contains 256 shades of color. So there are 256 shades of red, 256 of green, and 256 of blue. Each channel is 8-bit, remember? The 3 channels put together create 24-bit color (3 channels times 8 bits).

Red Channel

Green Channel

Blue Channel

Full RGB Image

The three channels combine to create the RGB color image.

In an RGB analogy, it's as if there are three transparent "floors" (channels) overlapping each other. Each of the 256 colored tiles on one floor mixes with the colored tiles on the other floors. The combination of 3 different "floors," each with 256 levels of colors, makes over 16.7 million possible colors.

An excerpt from The Non-Designer's Scan & Print Book

-- Web Color Reference --

(Click on the poster for closeups or to order.) Link to www.visibone.com

This 18" x 24" poster helps you make the most informed color choices within the web-safe palette. All 216 standard colors are grouped by their hue and reveal every shading series (light-medium-dark) in the palette. The color chips show the codes used by web and graphics design tools: hexadecimal HTML and decimal RGB.