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Working with Color Modes
Color modes define the colors represented in the active document. Although you can change the color mode of a document, it is best to select the correct color mode at the start of the project. Photoshop's color modes are Bitmap, Grayscale, Duotone, Indexed Color, RGB (Red, Green, and Blue), CMYK (Cyan, Magenta, Yellow, and Black), Lab, and Multichannel. See "Selecting Color Modes and Resolution" on page 15 for information on best use for each color mode. The number of channels in an image depends on its color mode. For example, CMYK image contains at least four channels, one for each color.Color modes determine the number of colors, the number of channels, and the file size of an image. For example, a RGB image has at least three channels (like a printing plate), one for each red, green, and blue color information. Color modes not only define the working color space of the active document, they also represent the color space of the output document. It's the document output (print, press, or monitor), which ultimately determines the document color mode. Color modes do not just determine what colors the eye sees; they represent how the colors are mixed, and that's very important because different output devices use different color mixes.Therefore, when selecting a color mode, know the file format of the document, and where it will be used. An image taken with a digital camera, and then opened in Photoshop would most likely be in the RGB color mode. An image displayed on a monitor would be RGB, or possibly Indexed Color. A photograph scanned on a high-end drum scanner would most likely be in the CMYK color mode. An image being sent to a 4-color press would be CMYK too. If you were creating a Photoshop document from scratch, the color mode chosen would represent the eventual output of the document, such as a Web page, inkjet printer, or a 4-color press.Switching Between Color Modes
Unfortunately, images do not always arrive in the correct format. For example, you take several photographs with your digital (RGB) camera, but the images are being printed on a 4-color (CMYK) press, or you want to colorize a grayscale image. Changing color modes is a snap, but changing the color mode of an image isn't the problem. The problem is what happens to the digital color information when you change color modes. For example, if you open an RGB image with the intent of sending it out to a 4-color press (CMYK), the smartest course of action is to remain in the RGB color mode through the processing of the image, and then convert the image into the CMYK mode. The reason is in how Photoshop moves between those two color spaces. For example, if you move a color-corrected CMYK image into the RGB color mode, and then back to CMYK, the colors shift because Photoshop rounds color values during the change process. Not to mention that a CMYK image is 25% larger than an RGB image, and the RGB color mode represents the color space of your monitor, not CMYK. It is impossible to view a subtractive CMYK color on an RGB device. If, however, the image originally came to you as a color-corrected CMYK image, then stay and work in that color mode. See topics in this chapter for specific steps to switch between color modes.
