Men? Women? What are you looking for?
 
 #8 [yellow] is safe.
 
 #11 [yellow/green] for men if you want that look.
 
 Nick

Filters for black and white are customarily divided into two
categories:

Correction filters - filters designed to make the colour rendering
of film more natural. Examples are light or medium yellow and
yellow-green filters.

Contrast filters- filters which make strong colours show up
as contrast differences. Examples are the tricolour red, green
and blue filters, and deep yellow and orange filters. You do
not normally want a contrast filter for pictures of people.

The most common correction filter is the medium yellow
(Kodak Wratten #8 / old Wratten K2). B&W film without a filter
records blue and violet as brigher than the eye sees them.
The medium yellow filter corrects this so that strong blues
and violets are recorded correctly as darker colours than
yellows and greens. In most cases this is a fairly subtle
effect unless there are strong blues present in the scene.
The most obvious effect is a slight darkening of blue sky
which makes clouds show up better. A #8 filer cuts out about
half of the light that the film sees under daylight even
though the filter is visually quite light.

Another common correction filter is the yellowish-green
(Kodak Wratten #11 / old Wratten X1). It has about the
same effect on the blue-violet end as the #8 filter,
but also filters out some of the red. It was originally
designed for films with excess red sensitivity. It is
useful if you want to make reds a bit darker than they
normally appear, or if you want to lighten the colour
of green leaves without being too dramatic. On a portrait
it will tend to make freckles and facial blemishes show
up more than they would with no filter or with a yellow
filter. The #11 filter has a filter factor of 4 with
most films: it filters out 3/4 of the light that the
film sees. There are weaker versions available from
some filter manufacturers: Hoya calls their weak version
the X0. I find the colour rendering with the weak
yellow-green filter to be very natural looking.

Incandescent light is rich in red and deficient in blue.
This is also true of daylight around sunrise and sunset.
This means that film shot under incandescent light makes
red appear too light. The #11(X1) filter is also useful
for this, but it requires an exposure increase of four
times (two stops) under conditions where there isn't much
light to begin with.

One thing to keep in mind is that if you do not know
why you need a filter, it is probably safest to use
no filter at all. The colour rendering of modern black
and white films is pretty good without a filter. Going
filterless has the additional advantage that it allows
you to use a higher shutter speed than you could use
with a filter.

Nick's advice is very good. The yellow filter is a very
safe choice, but if you are starved for light (as sometimes
happens) don't be afraid to shoot without a filter.

Peter.

Yes, a yellow filter can discriminate against freckles and some
blemishes.  This is an old trick.  Also, a soft focus filter is another
good thing to use for portraits.

You don't need any filters for portraits, unless there are some skin
problems (redness, pimples). In that case, a yellow filter can be of
some value.