While "sky darkeners" are the most popular, (for good reason), filters of all colors can come in handy under the right circumstances. Sometimes tones that are easily contrasted by their color, become nearly identical grey tones when rendered in B&W. Just remember that a filter lightens its own color and darkens the others. A Green filter will lighten Green foliage while a Red filter, because of its high Magenta content, will darken it. Take a B&W shot of a Red flower against Green leaves and you can decide whether you want a dark flower against light leaves or vice versa. Of course there are other things that play into it but that what makes photography a life-long pursuit

Here's an example, I shot this without a filter, took it home a souped it and didn't like it. The faded Red paint and the weathered wood, while clearly distinct by color contrast in the original scene, were rendered as nearly identical tones in the neg and print. I ran to the nearest camera store and bought a Green filter and reshot. The filter darkened the Red paint and gave me the separation that I wanted.

It is also sometimes used for outdoor portraiture where it will lighten the foliage and darken the skin tones slightly.