Specifically for black and white. Colours are not very precise; hence I'm giving Wratten numbers. These do point up the variations in (e.g.) the definition of "yellow".
Yellow
This is the most commonly used filter, and brings out white clouds in a blue sky to the amount that we would accept as natural. When most photographers used black and white film, a yellow filter was often left permanently fixed to the lens, as UV or skylight filters are today.
Like all the filters, it is available in lighter or deeper shades, with different filter factors, around the 2 or 3 times. One particular and useful variant is the “minus blue” version, which effectively reduces the blue and gives perhaps the best approximation to a sky with white fluffy clouds (when the sky is blue and has clouds).
One thing to keep in mind when using filters is that any parts of the scene illuminated by skylight rather than direct sunlight are being lit by blue light; and the more blue light you filter out, the darker the shadows will become (as we saw in the example above), and the greater the contrast between light and shade. This can be used with advantage to increase the rendering of the texture in the snow in snow scenes.
The effect on skin tones will not be too great; light skin and hair will be lightened, but blue eyes will be darkened. Freckles will be subdued, but not removed.
There are several different yellow filters, descriptively called light or pale yellow through to deep yellow, and with Wratten numbers 2A, 2B, 2C, 2E, 3, 5, 6, 8, 9, 12, and 15. No 12 is the "minus blue" we just referred to, and effectively cuts out blue light. It's a popular choice amongst black and white photographers.
Orange
This works in a similar way to yellow, but with more noticeable effect. Some say that this is the strongest filter you can use on a sky and still have a natural look to the result.
Other effects are the lightening of sandstone, and it can improve the rendering of wood grain.
As the filter goes more towards red, used in portraiture it will hide skin blemishes, but also lighten lips.
This is the first filter that can have an unexpected result with foliage. Despite the visual appearance of green, leaves do actually start to reflect strongly in the far red region of the spectrum; an effect that is clearly shown in infra-red photography, where foliage appears white. With a dark orange filter, foliage can begin to be lightened, an effect called the "Wood effect" after Robert W. Wood, who pioneered both ultra violet and infra red photography.
Wratten numbers are 16 for a yellow-orange filter, 21 (orange) and 22 (deep orange).
Red
Use this to give black, dramatic skies; and watch out for the blocked shadows that can result from the reduction of the skylight illumination. Coupled with slight underexposure, this can be used to create a moonlight effect.
A red filter also gives the greatest haze penetration. The difference between two otherwise identical photographs of a distant landscape, one with a red filter and one without, can be dramatic.
As the filter moves more to red, so the effect on foliage will be greater. It is possible to have foliage lightened to a very considerable extent using a red filter.
Wratten numbers are 23 (light red), 24, 25, 25A, 26 and 29. There is also a magenta (or "minus green") 32.
Green
Green filters get a mixed reception, with some photographers finding no use for them, and others making extensive use. The obvious effect is to lighten foliage in a scene, which can affect the mood of a landscape. Less obviously, it aids in the differentiation of greens, particularly in spring and early summer, when the difference between greens in foliage can be most pronounced. Used in portraits, it will darken skin tones and accentuate freckles and skin blemishes.
Wratten numbers 56, 58, 61, with 44 and 44A as blue-green.
Blue
This can be used with pan film to simulate ortho material, but why would you want to? Well, ortho was a popular choice for the “Hollywood portraits” of the 1940s. It darkens the skin (usually best used for male portraits) and lips, and has a look all its own. The darkening of lips was a problem with the early cinema films, and blue lipstick was used to make the actors appear more natural.
A blue filter will accentuate skin blemishes, so needs care on the choice of subject if this is not to create a very unflattering image.
And it will also act in the opposite way to a red filter, in increasing haze. This can be an advantage in some scenes, where the recession of tones enhances the feeling of depth in the photograph.
There are several different blue filters, but they are less commonly used. Wratten 38A, 47, 47A, 50 are some of them.
