Ussually I don't bother with filters for B&W portraiture, but here's a quick run down on what each tend to do
Red: lightens skin, gives kind of a porcelyn look. Because most blemishes are red in tone it lightens them, making them less noticable.
Yellow and Orange: Similar results to red but less, Orange is stronger than Yellow
Green: Will tend to deepen skin tones, and enhance things freckles, I think it looks someone with a tan. Not ussually recomended for portaiture, but there are times when it's useful (think Maine'ers in Feb.... we're a pale bunch that time of year).
Blue: Will act (sort of) like ortho film, lips will be dark, freckles etc will stand out. Never used this one myself, so can't say too much about it.
Peter