In B&W photography filters are used to change how the colors in a scene are mapped onto the various shades of black, white and grey. Their use is often a bit of a mystery to many photographers. Typically for landscape photography, a yellow filter is used to darken the blue sky, thereby creating a greater visual separation between the sky and the white clouds. A green filter is often recommended to help separate the various shades of green when photographing foliage. These are standard rules of thumb, but why do they work?
Let us consider a standard color wheel.
When discussing color, it is convenient to think of every color corresponding to a combination of three “primary” colors; red, yellow, and blue. Orange, for example, comes from the combination of red and yellow, while green results from combining yellow and blue. For convenience lets divide the color wheel into warm colors on the right, and cool colors on the left, and “complementary” colors are defined as being located on opposite sides of the color wheel. So green is complementary to red.
Filters work by transmitting some light and absorbing other light. A warm filter transmits warm light because it absorbs the cold light, only leaving the warm light. Filters function by being subtractive of complementary colors.
In addition to orange light, an amber filter mostly transmits light between red and yellow on the color wheel, while at the same time blocking/absorbing much of the complementary colors such as violet and blue. Using a light red filter (vermilion on the color wheel) in a landscape photograph will darken the tonal relationships in the sky, since the sky is essentially teal. It will also significantly darken the blue and green tones in the scene. If I want detail in my vegetation or the sky, this may not be what I want.
So what filter would best bring out detail in the red rock found in the American Southwest? A photographer mentor of mine (Nathan Mccreery), prefers to use a green filter. He says that a #61 dark green filter exaggerates the tonal relationships between the reddish tones of the sandstone and the yellow and grays that are also present. Green, being equal parts yellow and blue is unique in that it contains both warm and cool colors. In this instance, the green filter will render the yellow in the sand stone as lighter than the red, helping to achieving greater separation of tones in the rock.
To use filters to your advantage you must understand color; what colors are complementary (opposites) and what colors are similar.