Allows you to alter the name, color and fill opacity of the new channel.
The fill opacity is useful for setting how visible the underlying image is when you work with a channel that will later be converted to a selection. The advanced user will also use it to "control" ink opacity when he works with things like duotones. Also you have the opportunity to set a color which the channel will use when it's active.
The color field is a multifunction widget — doubleclicking with your left mousebutton will show the color selector and you can drag and drop colors from any color field in GIMP.