This expands upon the ideas given in post #18
With respect to black-and-white photography, yellow filters can enhance foreground contrast in a general way, provided that the scene is lit by the sun in a clear blue sky and that there are shadows present that are at least partially illuminated by the open blue sky. What happens is that the shadows generally receive little or no direct light from the full-spectrum sun, but usually are illuminated by light from the broader blue sky. The shadows are rich in blue light.
When we photograph the scene without a filter the blue light from the shadows is recorded on film strongly enough so that the shadows in the positive image are weak to moderate. But a yellow, orange, or red filter absorbs the blue light from the shadows, giving a weaker recording of the shadow areas on film. This results in a positive image with darker shadows. Since most scenes contain many tiny shadows, the result is greater contrast between the sunlit and shadowed areas. This can enhance the overall contrast by increasing the contrast of textured surfaces, such as sand, snow, gravel, the surface of brick, stone, concrete, grass, and other foliage, any shadow at least partially lit by the open blue sky.
There is a good example photo of this idea in the Kodak Workshop book, Using Filters. It shows two otherwise identical close-up photos of a maple leaf standing nearly upright partially embedded in snow in sunlight and under a clear, blue sky (sky not visible in photo). In the first photo taken without a filter the leaf casts a weak shadow and the snow appears as an amorphous mass of white with almost no texture. In the second photo using a Kodak Wratten #15 deep yellow filter the leaf casts a pronounced shadow and the texture of the many tiny “hills and valleys” of the surface of the snow are obvious making it quite 3-dimensional in the positive image. This photo is much more interesting and satisfying than the first one.
We already know the role of yellow, orange, and red filters in darkening a blue sky. But the darkening of shadows that are at least partially illuminated by the open blue sky to enhance foreground contrast by using the appropriate filter is often overlooked.
The Kodak Book Using Filters is a useful guide and can still be found used, and sometime as new old stock. The price is usually modest. These can easily be found using a search string, such as “Kodak book Using Filters” in Google. The book gives good explanations and with/without example photos to illustrate the ideas. Here is one such result:
Dead Link Removed