As noted by Brian Shaw a UV or Skylight will not have any noticeable impact on contrast, the UV might help with flare and increase sharpness with scenes at a distance. A skylight is intended color to reduce the blue cast, and not by much, sort of a color warming filter. A polarizer filter will darken the sky with panchromatic black and white film and bring out the clouds. Yellow to Red will darken the sky but also darkening foliage. And remember for a yellow to red filter to work there must be blue in the scene, as in sky or water or plants. A green filer will also darken sky, and is used to correct tones of caucasian skin tone.