The light sensitivity of photographic paper--that is, its ability to pick up an image from an image projected onto it from a negative and retain it--seems baffling unless you understand what is happening to the paper on a chemical level. The presence of certain chemicals on photographic paper affect how an image appears, how fast it takes the image to appear on the paper, and even whether the paper will accept a color or black and white image.
It is the layer of chemicals coating photographic paper that provides light sensitive properties. Silver halides--or the compound that is created when silver is combined with one of two types of silver fluorides, chloride, silver bromide or iodide--are suspended in gelatin. Depending on the particular mixture used to create the silver halide for a paper emulsion, according to R. E. Jacobsen (et al), author of "The Manual of Photography, the "speed, rates of development and fixation, image colour and tone reproduction qualities" are affected.
The presence of trace elements, like sulfur for instance, within the gelatin used in the emulsion can affect the light sensitivity, and often increases it. Silver halide crystals present within the emulsion have light-sensitive properties; when they are exposed to light, the surface changes, or develops, and is ready for processing. The crystal actually becomes what is referred to as "developable," or ready for fixing (the actual development process) in a chemical bath that stops the development of the crystals. Were they to develop fully, the image surface would turn completely black. Portions of the chemical coating (emulsion) receiving the most light develop silver halide crystals more densely. Much like film has a speed (ISO 400, for example), photographic paper does also. The paper speed refers to the rate at which the silver halide crystals develop--how light sensitive they are.
Monochrome and Color
Whether a paper is intended for monochrome (black and white) images or for color, largely depends on the levels of halide present in the paper's emulsion. The more bromide is present in the paper, the greater the sensitivity of the crystals to longer wavelengths, or those that can pick up color. There are photographic papers made especially for picking up the right black and white tones from color negatives. Photographic paper made for developing prints from black and white images will not process the tones from a color negative correctly, or directly translate the tonality into a grayscale image. Rather, the image will appear murky and out of focus.