You can view red, green, blue, and alpha channels—together or separately—in a Footage, Layer, or Composition panel by clicking the Show Channel button  at the bottom of the panel and choosing from the menu. When you view a single color channel, the image appears as a grayscale image, with the color value of each pixel mapped to a scale from black (0 value for the color) to white (maximum value for the color).

To see color values displayed in the channel’s own color instead of white, choose Colorize from the Show Channel menu.

When you preview the alpha channel, the image appears as a grayscale image, with the transparency value of each pixel mapped to a scale from black (completely transparent) to white (completely opaque).

All channels, colorized color channels, and alpha channel

You can view other channel values, such as saturation and hue, by applying the Channel Combiner effect and choosing Lightness from the To menu.