The Color Exchange filter performs color replacement on user defined colors. By selecting a "From color" from the image window within the Color Exchange dialog window, and then selecting an appropriate "To Color", the selected color will be replaced.
The From Color settings include three threshold settings that can be adjusted to include, or exclude, more of any of the three base colors, RGB. Increasing the threshold increases the agressiveness of the filter when the filter seeks matching colors. That is to say that a higher threshold will result in more colors being selected based on their similarities to the adjacent pixel color. The Lock Thresholds checkbox locks the three threshold sliders together to give a uniform threshold across each of the colors.
Once the color selection is made, clicking OK will replace the selected colors with the color chosen as the To Color. This filter can be extremely useful for replacing colors quickly and easily.