A common beginner mistake is to think this is just about darkening blue sky for sake of better clouds. But how is your choice of filter going to affect other things in the same scene? Go to the Southwest where there is a lot of brick-red Navajo sandstone and related soil color, use a yellow, orange, or red filter to darken the sky, and you'll get over-exposed bland paste-like rock tones. Same with brick buildings. Use a green filter instead, and you'll not only bring out the clouds in a blue sky, but deepen the brick hues. If you simply look through a given color of contrast filter, you can get a general impression of the result, even though pan films see things somewhat differently than our eyes do.
Just a few days ago I went down to our shoreline, and had to remove my routine deep orange filter and replace it with a medium green one. Why? Not only was there a blue sky with some interesting clouds in it, but blue salt marsh pools in the foreground reflecting all that. And all around them was a lot of low salt-marsh foliage, turned red in autumn. If I had used an orange filter, everything surrounding the pools would have been rendered nearly as light as the pools themselves. All the drama in the scene I wanted would have been ruined.
The secret was the green filter instead. The blue of the sky and those reflective pool was somewhat darkened, allowing clouds and cloud reflections more opportunity to show, while the red foliage itself was dramatically darkened, making the pools themselves, and their details, far more apparent.
Learn to think and see like film, and not just along the lines of some filter advertisement.