When you open an image, the composite channel is selected, and that's where you do most of your editing to an image. When viewed individually, channelseven color channelsare displayed as grayscale images. You have the preference of viewing your color channels in color, but this is not as useful as it may sound (see the following section, "Using the Channels Palette").
Those of you into Photoshop numerology will be interested to know that you can have as many as 56 channels (57 including the composite channel). Common sense, however, will ensure that you use far fewer. Adding channels to your document will increase its file size, although not significantly; the more important issue is that too many channels will likely cause confusion. By default, bitmap, grayscale, duotone, and indexed color images have one channel; RGB and lab images have three; and CMYK images have four. Alpha channels are saved with the document.
This "white: editable; black: masked" relationship can be reversed in your Channel Options. Don't worry about it: what's important is that the channel defines a separation between the areas of your image that you want to affect and those that you want to protect. If it turns out that your alpha channel actually defines the opposite of what you need, then either Invert the channel (Image > Adjustments > Invert or Command/Ctrl-I) or, if the selection is active, make an inverse of the selection by choosing Select > Inverse or Command/Ctrl-Shift-I.
In addition to the built-in color channels you can, throughout the course of editing an image, add additional channels, called alpha channels. Despite the highfalutin name, these are nothing more than saved selections. Because they are saved selections, alpha channels can be recalled, viewed, and edited at any time. Alpha channels do not print.
Once you've gone to the trouble of making a selection, it's a good idea to save it as an alpha channel (Select > Save Selection...), so that you don't need to make the selection all over again the next time you want to edit that same portion of your image. Alpha channels are also referred to as masks. But whether you're calling them alpha channels, saved selections, or masks, they are the same thing: a grayscale representation where the white pixels represent the editable (selected) areas, the black pixels the protected (unselected, or masked) areas, and the gray pixels, if there are any, the partially selected areas.
A third, less frequently used type of channel is a spot color channel, used to specify an additional plate for printing with spot color inks. When it comes to printing on a printing press, Photoshop is essentially a four-color program; that is, when preparing images for printing on a printing press the colors in your image are made up of varying percentages of cyan, magenta, yellow, and black. If you need to print with specific spot colorseither because your job will be printed on a low budget, for example, a two-color print job that uses black plus a second specified spot color, or because you need to accurately match a color from a corporate logo or brand imagerythen you need a spot color channel.
Not all file formats support alpha channels. JPEG, for example, does not. To save your alpha channels with your image, save your file in Photoshop, PDF, PICT, Pixar, or TIFF formats. To save spot color channels, use Photoshop or DCS 2.0.
Figure 2.2. A simple spot color image (example A) designed to print in only two colors (example B).