Pantone
Pantone Colors. The Pantone Matching System (pms) describes a huge range of colors for printing. Each color corresponds to a specific ink that can be used, and many of these colors can be closely approximated in 4 color process printing. (Note "many" and "approximated"; some pms colors can not even be approximated by 4 color process and many can not be exactly matched).
When you are picking a color you want, say for the background of a cover, the pms colors are convenient references, as long as you realize (1) what you see on your monitor is not exact, and (2) what can be printed in 4 color process does not exactly match the pms color. But it's better than having to say "you know, a kind of pinkish red that's not too light or pale".
You can see Pantone colors on the web here and Pantone has an ap for your iphone or ipad. Generally you'll be interested in the guide for "coated" paper. There's another ap that I like better, Lanse, which allows you to capture colors using your phone camera, or to sample using color mixing. Another ap, Color Expert will also show you the best 4 color process approximation of each pms color.