John O'Keefe-Odom , Feb 16, 2009; 08:22 p.m.
Yellow works well with people. A strong red will do a great job of increasing contrast to emphasize shadows. Red and oranges will do well with the sky, because the sky is blue [orange is opposite of blue]. Blue filters do a great job with glass and varnish and other shiny surfaces [might do well with a glassy skyscraper or windows on the street]; the only major detraction of the blue filters is that they are usually strong, and will have a big filter factor [often 8X]; they'll require longer exposures.
However, there will be two basic rules for black and white filter selection; they're really simple.
1. To emphasize a subject; look at its color; pick a filter that's close in color.
2. To reduce, "knock back" or "play down" a subject; look at its color; pick the opposite.
To check, simply hold the filter up to your face, and flash it into your line of sight for a second.
For like and opposite, think of a color wheel. Organized clockwise, the wheel would go like this, from 12 o'clock towards 3 o'clock and all the way around: Purple (2 o'clock), blue (4 o'clock), green (6 o'clock), yellow (8 o'clock), orange, red (11 o'clock). Imagine that color wheel, learn it and keep it in your head; all kinds of things related to color will be easy to remember and quick to understand.
For example, if you were about to take a picture of plants; you'll see a lot of times they have grayish midtones to the eye; to "smooth" out their color and tone, and make it look more like a green leaf "should", use a green filter. To make the plants look more silhouette, or high contrast, try a red. The correct filter color will be based on the colors you see. The strength or intensity of the filter will depend on how much of a change you want to make. Hope that helps. J.