In addition to the previous post, we will examine how to "bleach bypass" in Photoshop.
Bleach bypass, also known as skip bleach or silver retention, is an optical effect which entails either the partial or complete skipping of the bleaching function during the processing of a color film. By doing this, the silver is retained in the emulsion along with the color dyes. The result is a black and white image over a color image. The images usually would have reduced saturation and exposure latitude, along with increased contrast and graininess. It usually is used to maximal effect in conjunction with a one-stop underexposure.