Selecting by Color Channel
Often, when you need to make a selection, using the full-color image isn't the best starting point; one of the color channels would be more useful. When you open a digital photo in Photoshop, it appears in RGB mode, which means that it's made up of three primary colors: red, green, and blue. There are color channels for each of these colors, which you can view by choosing Window > Channels to display the Channels palette. The composite RGB channel is selected by default, but you can select the Red, Green, or Blue channel individually to preview the image data that's stored for that channel. These appear as grayscale images, as shown in Figure 8.
Figure 8 Check the Red, Green and Blue channels before making a selection—one of these might be a better choice than the composite RGB channel as a workspace.
Often, you'll find that one of these channels contains the data you're interested in selecting, and it's easier to make the selection on the channel than in the full-color image. When this is the case, select the channel you want, choose a selection tool from the toolbox, and make your selection on that channel. When you're done, reselect the RGB channel to return to the full-color image, and work with the selection as you would any regular selection.
Make it a habit before you make a selection to check the Red, Green, and Blue channels in the Channels palette to see whether any of them show the area to be selected more clearly than in the full-color image.