The Channels tab displays the three RGB channels, showing the current red, green, or blue color values of each pixel in your image. The RGB channel thumbnails are grayscale representations of each color channel with white representing 100% color and black representing no color.
The RGB channels each have an eye icon, so you can look at your image in a single color channel. Click off the eye icon in the Blue and Green channels, so that only the Red channel is visible. Bright red in the red channel is the equivalent of a maximum red value for that pixel; black means that the pixel has no red at all in it. You will see bright red for areas that are white or bright red. If all three channels have maximum values for an area, that area is white in the actual image.
The RGB channels are always active when a layer is active. They display the color values of all visible layers, not just the active one. Unlike layers, the RGB channels can all be active at the same time. You can also choose to work in one or two specific color channels by clicking on the appropriate channels to activate the ones you want and deactivate the ones you don't want.
You can create new channels by clicking the button. This will bring up the New Channel dialog where you can set name, color, and fill amount of the new channel. If you want to alter the values later on, you just double click on the channel which will bring up the Edit Channel Attributes dialog. Normally you don't work with extra channels, but they can be very handy for creating and storing selections.
If you right click on the layer name you will be able to access the layer menu. In the layer menu you have access to several other layer commands, see Chapter 6.
An experienced user can also use them to create patterns and advanced colored images. The channel stack tools and are more or less only useful when you work with those kind of images.
You are able to store a selection as a channel with the rightclick Select Save To Channel command. This creates a new channel where you can paint, erase, and perform any other drawing function. The altered channel can be turned into a selection again by clicking on the Channel To Selection button. This is a very convenient way to alter selections and store several selections. In fact, you can create selections from scratch by creating a new channel, altering it, and then applying Channel To Selection.