The late Ansel Adams was an advocate of filters for his dramatic black and white skies and his shimmering forests. We learned from Adams that filters of yellow, orange or red could progressively darken skies, that a green filter could bring out details in the foliage of deep woods. Unfortunately, we also learned that a yellow filter used on color film created a jaundiced yellowish-looking photo. For instance, the Contrast Red filter creates dramatic dark blue skies and lightens reddish hues without altering other colors in the image. The image on left is directly from my camera.
Middle image shows polarizing effect.
Image on right shows polarizer plus contrast red which results in noise. Even more amazing is the Polarizing filter, something previously though impossible by digital photographers. Still, it's a wonderfully useful tool for creating natural looking polarized effects. The image at the top of this column illustrates the difference with the inset photo being the original rainy day photo and the larger showing the sunshine effect. All of these filters can be used in mixed combination with some very unusual results, often the addition of unwanted noise.