They are not needed, with the exception of infrared films.
However they may be helpful by changing the reproduction of colours into shades of grey.
Basically an object of the same colour as the filter will be lighter in relation to the other colours in the final positive image.
As the filter will absorb light from the other colours and grey objects (and even a bit of the light from the an object of same colour) you may consider an exposure correction, depending on the final effect you desire: lightening that one colour or darkening the others.
This is put very simply. Have a look into one of the basic photography books.
A thumbnail guide - light yellow to draw out clouds by darkening a blue sky. Next, a darker yellow, for a more pronounced effect. Finally a red for stark white clouds against near black sky.
Closer to the ground, a green is good for separating red and green in foliage and flowers; many foliage colours translate to almost the same shade of grey.
I keep a light green/yellow for better portraits of caucasian skin tones under tungsten light. I also have a light orange for when I want to emphasize freckles, etc.
Like the others have said, I would read up on the subject before you start buying filters.
Michelle if you want to shoot landscapes then the yellow or orange is important to bring out the skys, yellow for a normal looking sky or orange for a dark, menacing sky, I take black and white landscapes as part of my living and these are the only 2 filters I use, for portraits or general studio work you really don't need them, although if you are taking b/w pictures of wood with a nice grain or texture a green filter can help bring out the texture/grain.although not the textbook awnser for all practical purposes that is really all you need to know, best way is to get a set of black and white filters and see what they do,look in the sale bins for red yellow green and red, but don'tforget to add 3 stops for red,2 for orange and green or 1 stop for yellow, Richard
but don'tforget to add 3 stops for red,2 for orange and green or 1 stop for yellow
Just to avoid potential confusion; depending on your camera you might not have to make a concious adjustment of the exposure. If you camera is an SLR or other camera that has TTL metering (through the lens) then it will see through the filter and make the necessary adjustment automatically.