The process that creates a picture on a piece of film involves a reaction between light and the sliver bromide crystals spread through the gelatin of the emulsion layer.
A negative is born when millions of exposed silver bromide crystals are converted to Silver metal by the developer.
Silver bromide crystals are light sensitive, they don't respond equally to all wavelenghts of light. Colors are actually different wave-lenghts within the visible part of the electromagnetic spectrum, these wavelenghts must be factored in when making Black-and-White Photos. Different types of Film responds color differently. Orthochromatic film responds well to shorter wavelenghts, toward the violet to yellow end of the spectrum. Panchromatic Film gives more of a natural appearance. This film records most all colors, red through violet. Infra red Film only records visible colors as well as some longer wavelenghts that are not visible. Its' toward the ultra-violet end of the spectrum. Unless the silver bromide is properly exposed, they will respond variably to shorter wave lenghts, starting at ultra-violet through blue green.