If you are working on scans on black and white art or images in Photoshop, you have likely come across the problem of murky scans where the paper is off white or the larger areas of dark in are not uniform. When printed these become real problems as the edges of lines are softened and definition is lost. If you are wanting to colour black and white art, it also interferes with any multiplied layer as the off white colours bleed up into the overlay.
The obvious solution would be to select the light tones and lighten them, and select the dark tones and darken them, this is what so many Photoshop artists do because it increases the contrast dramatically, whitens the whites and blackens the blacks. There is a better way though, and it takes just a few minutes to learn it.
If you use the Select Colour Range option, and then alter the Brightness and Contrast options, what you are telling Photoshop to do is ignore mid-tones, and this heavy handed approach will lose any nice feathering or fine lines that you original scan had. The likelihood is that if your scan requires some work, it isn’t because it needed all the fine lines destroying, what it needed is the paper lightening without losing those finer lines, so instead of using the Brightness and Contrast tools, try instead adjusting the image levels at their base.