Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 327

transparency attributes. When you're working on images with indexed colors,
a single channel called Indexed, which usually won't possess an alpha channel,
will replace the three main channels. These channels cannot be renamed.
You can make each of these channels visible or invisible by clicking the
eye icon. The visible colors in your image will be modified accordingly.
By clicking a channel in the Channels dialog, you can set it to active (blue)
or inactive (white). When you edit an image, your changes affect only the
active channels. Setting a channel to inactive will ensure that any subsequent
changes to the image will not affect the channel.
In contrast to the Layers dialog, in which only one layer can be active at a
time, the Channels dialog allows you to activate more than one channel at a
time. In fact, when you work on an image in full RGB view, all color channels
must be active.
An image can have more channels than the three color channels. These
additional channels are displayed in the bottom half of the Channels dialog.
You can create these channels yourself. For example, you can select the
Select > Save to Channel menu item to create a channel from a selection. This
channel will then be listed in the Channels dialog as a custom channel (black-
white image) and saved together with the image (but only when saving in the
XCF format!). You can then load your custom channel whenever you want and
create a new selection from it.
Figures 4.19, 4.20, and 4.21
The single color channels (of a black-and-white image in RGB mode) and their representations in the image window.
An RGB representation does not necessarily mean that the image is colorized.