They are called contrast filters, and people generally think of them that way. Esp when just shooting landscape since most of the colors tend to be the same. The sky is usually blue, trees are usually green etc. I work in the graphic arts field, and had the importance of color filters impressed on me in the days when we did color separations for CMYK press work. A red filter passes red wavelengths, but at the same time blocks green and blue. Therefore, it will give MORE exposure to red objects and less exposure to green and blue objects. Thats why a deep red filter darkens the sky in landscape photography. It does not allow as much blue light to pass, making anything that is blue appear darker.

If you look up any list online or in a book, it will typically tell you what the uses for a filter are. Red is always listed as darkening skys in landscape. Going by what wavelengths we know a filter blocks and transmits, we can make use of filters in other ways. Caucasian skin has a reddish tone, so red, and orange filters will help lighten skin tones varying degrees depending which filter you choose (and subsequently which wavelengths of light you decide to block).