Monochrome film doesn't have a linear colour response, and blue tends to be brighter than red. But then the photographer uses a colour filter to cut back on the bluer light. But RGB colour doesn't really have the spectral resolution, the commonplace yellow (anti-blue) filter doesn't cover the same range of colours as the blue channel. If you have the colours distinct enough, you can probably select the lips in Photoshop, invert the selection, and convert the selected area (now not the lips) to greyscale. Photoshop Elements came with my film-scanner and certainly has the features to turn colour images into something close to black-and-white film, even adding grain effects.