Photoshop includes some very sophisticated tools for manipulating channels. You can use them to modify existing channels or even to create new documents from channels.
The menu command Image, Adjustments, Channel Mixer opens the dialog box shown in Figure 16.20. You can adjust the content of a color channel by using the content of the other channels as source material. In Figure 16.20, the gorilla is too red. The content of the channel can be modified by copying some of the information from the Green and Blue channels.