The Adjust FG-BG filter applies your current Foreground and Background colors to the active image based on brightness of the image. The filter determines the color gradient of the image based on the coloring of the image, and applies a gradient based on the Foreground, and Background colors.
The Color Range Mapping filter provides a dialog such that the Foreground and Background colors may be chosen. Colors for the source and destination may be selected.
A common use for this filter is repairing the color balance of faded photos. Opening a faded photo, and selecting a Foreground, and Background colors that are suitable for the photo, and then running the Adjust FG-BG, can produce desirable results.