This tool replaces the hue of every pixel in the image by the hue of the specified color. Only the hue is used, there would be no difference if you used (128, 0, 0) or (255, 0, 0) for the (R, G, B) values, because both have the same hue (red).
You may use it for whatever you wish 😉. Sometimes monochromatic images are used to indicate disabled state or as an artistic effect.
The operation is implemented using java script and it is very easy to modify it. If you have any suggestions regarding its functionality, please let me know.