Graphics Fundamentals: Distinguishing Photoshop Color Modes

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Each image you work on or create in Photoshop contains color -- even if the only colors used are black and white. To reproduce these colors correctly for the various me-diums, Adobe incorporated the most common, established color models, such as RGB and CMYK, into Photoshop. However, in order to use a particular color model for a particular image, you need to tell Photoshop which one you want to use. You do this by choosing a color mode. In order to choose the appropriate color mode for your image, you need to first understand how each one works. In this article, we'll describe the four basic color modes that are available in Photoshop.

Color Me This
The color mode you choose determines the number and range of colors that can be displayed in an image, as shown in Figure 1, as well as the number of available color channels. This also affects the image's file size; the more color channels there are, the larger the file size. To explain the different color modes, we'll first show you where you can set up or change an image's color mode. Then we'll describe the four basic color modes used in Photoshop: Grayscale, RGB, CMYK and Lab Color.

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basics_pscolor_fg1b.jpg
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Figure 1: Color modes affect what colors are visible in an image.

Accessing the Color Modes
There are two ways to access Photoshop's color mode options depending on whether you're opening an image or creating a new one. If you're creating a new image, you can choose a color mode in the New dialog box. To do this, choose File > New. Then in the Image Size area of the New dialog box, select the color mode you want to use from the Mode pop-up menu, as shown in figure 2, and then click OK.

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Figure 2: Select the color mode when you start a new image.

When you open an image in Photoshop, the color mode in which it was created is the default color mode for that image. However, you can change an image's default color mode by choosing Image > Mode and then selecting a color mode from the submenu, as shown in figure 3. As de-picted by the check mark next to the RGB Color option, the default color mode of our image is RGB. However, once we select the CMYK Color option, the color mode is changed.

Tip: Since an image's default color mode is marked in the Mode submenu, displaying this menu is a quick way to double-check an image's color mode.

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Figure 3: You can change modes on a current image through the Image menu .

Changing an image's color mode changes the appearance of the image's colors, as well as the image's file size. Now that you know how to apply a color mode to an image, let's examine each mode more closely so you can choose the appropriate mode for your image.

  • The Gray-scale mode. If you want to convert a color image to a black-and-white image, you can then apply the Grayscale mode. The Grayscale mode uses up to 256 shades of gray. Each colored pixel in an image is converted to a shade of gray that represents the pixel's luminosity. A grayscale image only has one color channel, as shown in figure 4.

    Tip: To view an image's color channels, choose Window > Show Channels.

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    Figure 4: Grayscale images have only one channel.

    To convert an image to grayscale, choose File > Open to open the image you want to convert. Switching between color modes can affect the quality and clarity of your original image. There-fore, you should work off a copy of your original by saving it with a new name. To do this, choose File > Save As, enter a new name in the Name text box, and then click Save.

    To apply the Grayscale mode, choose Image > Mode > Grayscale. When you do, Photoshop asks if you want to discard all the color information in the image, as shown in figure 5. When you click OK, the pixels are converted to various shades of gray, as shown in figure 6.

    basics_pscolor_fg5.gif

    Figure 5: Converting a color image to grayscale means you lose the color information in the file. Therefore always works of a copy of the orginal image. .

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    Figure 6: Because you lose color information when converting to grayscale, work off a copy of the original image.

  • The RGB color mode. The RGB color mode should be used for images that will be viewed onscreen, such as on a Web site or in a PDF file. Computer monitors display colors differ-ently than when an image is printed because they use the RGB color model. RGB images contain three color channels, as shown in figure 7, and are made up of different red, green and blue val-ues. For example, a pink color would be made up of the following RGB values:

    Red=225
    Green=54
    Blue=92

    To apply the RGB color mode to an image, open the image and choose Image > Mode > RGB Color.

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    Figure 7: RGB images contain three color channels.

  • The CMYK color mode.The CMYK color mode should be used for images that will be printed, such as for magazines or advertisements. This color mode contains four color chan-nels, as shown in figure 8, that represent the four inks used to print a color image: cyan, magenta, yellow and black. Each color in an image is given a value in each of the four channels. For example, to create the same RGB pink color mentioned earlier, the CMYK color mode would break down the color as follows:

    Cyan (C)=0
    Magenta (M)=79
    Yellow (Y)=47
    Black (K)=0

    basics_pscolor_fg8.jpg

    Figure 8: CMYK images contain four color channels.

    To change an image to CMYK, open the image and choose Image > Mode > CMYK Color.

  • The Lab color mode. The Lab color mode actually includes all of the colors that you can create in both the RGB and CMYK modes. Like RGB, the Lab mode consists of three color channels, as shown in Figure 9. The first channel is Lightness (L). The Lightness component, otherwise known as luminance, can range from 0 to 100. A Lightness value of 0 equals black and a value of 100 equals white. So, the higher the Lightness value is, the more vivid the color. The other two channels, a and b, represent color ranges. The a channel contains colors ranging from green to red and the b channel contains colors ranging from blue to yellow. The Lab color mode breakdown for our pink color is as follows:

    L=58
    a=77
    b=31

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    Figure 9: Lab images contain three color channels, but unlike RGB the colors are based on image lightness.

    Unlike RGB colors that are screen-dependent and CMYK colors that vary with printer, ink and paper characteristics, Lab colors are device-independent. Therefore, the visual characteristics of these colors remain consistent on monitors, printers and scanners. The Lab color mode is good to use when working with Photo CD images and when independently editing luminance and color values. It's also useful when you need to move images between operating systems and print to PostScript Level 2 and Level 3 printers. To convert an image to Lab color mode, open an image and choose Image > Mode > Lab Color.

A Colorful World
Working with color in Photoshop can sometimes be tricky. But knowing how the different color modes work and when you should use them is the first step to properly preparing your images so you get the results you want.

This story is taken from "Photoshop Fundamentals" (Element K Journals).

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