How I can remove all color areas from an image?
I don't mean desaturate. Desaturate turns color into gray. I want to turn any color into white, and preserve any existing grays and blacks.
For instance in old print engravings they would dab paint onto the engraving print. I would like to restore it to the original engraving part of it and remove the painted on parts. Or something similar with old pen & ink illustration when they would start with a black and white image and then a colorist would screen a few solid color areas on top of it later, I would like to be able to restore it to the original colorless drawing.
Example of an engraving, I would like to remove the red splotch and restore it to the original engraving: