There are several filters, used exclusively in the darkroom (as you print your image onto the light-sensitive paper) to increase/decrease contrast in a black and white photograph. To use them, you eject the small slide (just underneath the lamp, above the negative tray for most enlargers) and insert the filter of your choice.
You must compensate to some degree for the diffusing/light-absorbing qualities of any colored sheet of plastic, increasing the time the light is on and such.
There are a variety of photographic filters numbered from 0 to 5. Filter 0 adds very little contrast; filter 5 results in almost absolute contrast.