Although we see colors such as red and green very distinctly, they often have very similar appearance in a black & white image. This can result in a flat photo where striking contrasts are lost in the translation to gray. A result of both the hue of a subject and its tonal value.

This problem can be overcome with the use of color filters which allow analagous colors of light through to the film and block out complementaries. In short, they lighten similar colors, and darken opposite colors. 

Although as a color photo a red flower on a background of green foliage has strong contrast, as black & white the flower and foliage will convert to a similar gray, 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 gray, and the flower as a dark gray. Both filters create tonal differences and restore contrast between the subject and background.

Although they are available in a broad spectrum those most commonly used are yellow, red, and green. Exposure compensation is necessary when using color filters according to how much light they absorb (how dark they are).

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 gray