Black & White
Colour filters control contrast by altering the tonal values of grey
Although we see colors such as red and green very distinctly, they often appear very similar in a black-and-white image. This can result in a flat photo where striking contrasts are lost in the translation to grey. This problem can be overcome with the use of color filters which allow analogous colors of light through to the film, and block out complementary (opposing) colors. In short, they lighten similar colors and darken opposite colors, allowing the photographer to exercise some selective control over tonal values.
Although as a color photo a red flower on a background of green foliage has strong chromatic contrast, as black-and-white the flower and foliage will convert to a similar grey, so the flower will lack definition and blend in with the background. If a red filter were used it would allow the red light through the lens but block the green, resulting in a light flower on dark foliage. If a green filter were used it would do the opposite, admitting the green and blocking the red light, rendering the foliage a light grey, and the flower a dark grey. Both filters create tonal differences and restore contrast between the subject and background.
Below are details of the exposure compensation needed and the effects of the main colour filters used with black-and-white film.
YELLOW (1 stop) - subtle effect: lightens warm colours, darkens blues and prevents bright whites from blowing out
GREEN (2 stops) - lightens greens and darkens blues and reds
RED (3 stops) - lightens reds and dramatically darkens blues and greens to very dark grey