Many of us Utah types use an orange (15) filter with the rock. It darkens the sky significantly and lightens the rock a little. This is usually the effect you want. A polarizer can also be used to darken the sky without changing the rocks, or the two can be used together for a more dramatic effect, with skies like the 25 filter but not such light rocks. I sometime use a yellow green filter (13), but the orange filter seems to work better in the red rock country. The usual rules apply: things similar in color to the filter are lightened, things in the complementary color are darkened.