If you are looking for any specific rule there isn't. However, my personal decision to choose color over black and white is based on simple design principles. The basic elements of design can be brokendown into the following categories: point, space, shape, tone, form, mass, line, texture, pattern, time, light, and color.
When you are on location or in the studio for that matter and you are deciding whether or not to choose black and white over color, what you have to ask yourself is if color is a major design element in the person, place or object that you are photographing. If color is not a major design element in your subject matter, then shoot black and white, but again this is a personal preference, not a rule.
As an extreme example look at these photos by Sandy Skoglund, a very famous photographer and installation artist.
http://www.flickr.com/photos/sindestino/350561544/
http://www.flatironmag.com/skidoo/Sandy-Skoglund.jpg
http://imagearts.ryerson.ca/imagesandideas/pages/database/images/Skoglund_Sandy_w01.jpg
http://www.flickr.com/photos/disneyprincess/487522864/
So ask yourself, after looking at these photos, would these photos be the same if they were shot in black and white? If color is a major theme or element in your subject matter it makes sense to shoot in color.