it does matter, because a monochrome image does not have any CIE components (which are attributes of RGB images) - there's nothing to extract.
in this case, the monochrome image itself is its own lightness. if you want to mask your image with itself, just drag the name tab to the PI workspace and a clone of the image will be created. you can then drag the clone's name tab to the space below the name tab of the target image and the clone will be set as the mask.
rob