With standard pentachromic film yellow absorbs some blue and darkens the sky some, an orange absorbs more red and darken the sky more, red absorbs almost all red and darkens the sky to almost black. The darker the sky the more the the clouds will stand out. The trade off is a yellow, orange or red filter darkens green foliage. A yellow green or green will darken the sky some and lighten foliage, so good for light skins in portraits. Most introductions to black and white photography books will explain in greater detail. If you look under the exposure thread you will many posts on the subject.