Understanding what you're seeing in each channel gives you the know-how to create complicated selections and fine-tune your images. In this section, you'll look inside the different color channels, beginning with the most common image mode: RGB.
This section doesn't cover alpha channels. They're so important, they get their very own section, which starts on Creating an Alpha Channel.
Unless you're preparing an image that's headed for a commercial printing press (as opposed to the inkjet printer you've probably got at home), RGB mode is the place to be. After all, your monitor is RGB, as are your digital camera and scanner. But as the box on Why Are Channels Gray? explains, Photoshop doesn't display individual channels in red, green, and blue—they're in grayscale so you can easily see where the color is most saturated. Because colors in RGB mode are made from light (How Color Works), white indicates areas where the color is at full strength, black indicates areas where it's weakest, and shades of gray represent everything in between (see Figure 5-3).
No matter which color mode you're in, you can cycle through the channels by pressing ⌘-3, 4, 5, and 6 (Ctrl+3, 4, 5, and 6 on a PC) though you'll only use that last one if you're in CMYK mode, which has four channels instead of three, or if you're trying to select an alpha channel in RGB mode. To go back to the composite channel (The Channels Panel and You) so you can see the image in full color, ...