Understanding Color Channels
Photoshop’s Channels panel offers the capability to look at each of the individual color components of an image. Each image has one or more color channels; the number depends on the color mode chosen. A CMYK image has four separate channels. One in RGB mode has three, and grayscale images have only one channel apiece. Each channel holds information about a particular color element in the image. Think of individual channels as something like the plates in the printing process, with a separate plate supplying each layer of color. You can often create interesting textures or special effects by applying filters to just one channel. Figure 5.20 shows the Channels panel (twice) with RGB and CMYK channels.
Figure 5.20 In the RGB image on the left, I’ve selected the mask from an adjustment layer; in the CMYK image on the right, the composite channel is selected, meaning that all the color channels are active.
You learned about other channels called alpha channels, and their several uses, in Hour 3, “Making Selections.” Alpha channels can define the placement of spot colors (Pantone, for example). They also contain the maps for selections you create and want to save with the image to which you have applied them, as well as adjustment-layer masks. Hour 12, “Using Masks,” talks more about alpha channels.