I use color filters a lot. My basic filters are red (there are three types, red, deep red and opaque red), orange/yellow, green, blue. This leaves out color correcting filters which are made for color film, but can be misused for b&w effects, polarizers which do the same thing to b&w as they do to color, neutral density for aperture control, gradient for horizon mitigating, and a whole slew of special effects filters that do weird stuff.
Red (Wratten 25) makes most skies dramatic, makes feminine skin lighter and smoother and it hides blems. Red filters darken green (leaves, grass) and blue (sky, water), but lighten white and warmer colors like clouds and people. I often put a red filter on when I am shooting panchromatic film in daylight if I want a contrasty, dramatic look. A red filter dims light at midday by three f-stops.
Deep red (29) does what red does, but much more so. This is my filter of choice for IR film because I can see what I am shooting through a SLR. It comes very close to an opaque red (IR pass) filter with the old HIE film, and Efke's Aura film, but not with SFX. A deep red filter can reduce exposure by four or five stops, depending on the film and lighting (5 at midday, 4 in the late afternoon/early morning, or under incandescent light.)
Opaque red (89) is good with infrared or red sensitized film. Opaque red filters (actually not opaque, you can see the filament of a light bulb through one) usually add way too much contrast to normal b&w film, and are the filter most often used in digital infrared cameras. If you want the effect of white foliage with modern IR film, then an opaque red filter - or a deeper (#87, #93) is needed. Discover the filter factor by experimenting with your lighting, film and development.
Orange and yellow (#9, 12, 15 or 16) are similar to red, but they darken green foliage much less (except for infrared film, where they darken green more.) Orange adds a slight sense of added contrast, but yellow yields a very natural rendition of sunlight scenes. Under incandescent light (studio hot-lights included) a yellow filter adds brightness to skin, while downplaying most other colors. I often use a deep yellow filter (#9 or 12) (also known as a 'minus blue' filter) for feminine portraiture. It subtracts less than one f-stop of light from a given exposure, while an orange filter cuts one and a half to two stops.
Green (#58 or 61) will darken skin, especially ruddy, masculine skin. It will lighten (but not dramatically) foliage and leave sky and water looking normal. Green filters should not be used for feminine portraits intended to be flattering, as it will accentuate blotchiness, eye bags, wrinkles and blems. With men, these features add "character", so use one for those pictures of wizened faces of old men. Or of working men, or of anyone whose skin you wish to look darker. It'll take one to two stops away, depending on the subject.
Blue (#47, or the #80 series of light balancing filters for a milder effect) is a special effect filter. It adds haze, fog and other atmospheric effects, which increases with the humidity! It creates skin leisons , blems and blotches where none exist - it make zombies out of even the most beautiful people. And it adds contrast in almost uncontrollable amounts to landscapes - skies turn white, the ocean looks frothy, and bushes/trees become silhouettes. If you are into post-modern extrema, then this is the filter for you! Figure 4 or more stops are lost, depending on the effect you want.
