The Color Fill tool is used for replacing an original color with a major color by filling areas of similar color. The value of the Tolerance parameter is taken into account. The bigger this value, the wider the color range that will be replaced by the Color Fill tool. The tool does not require selecting a working area.
If the "All Layers" flag is turned on, the Color Fill tool fills the areas of an active layer with the colors found on all layers visible at the moment (not only with those on the active layer).