Rules for using the Colon:

Use a colon to set off a summary or a series after a complete main clause:

Example:  It is time for the baby’s birthday party: a white cake, strawberry-marshmellow ice cream, and a bottle of champagne saved from another party.  (Joan Didion)

Notice that a main clause does not have to follow the colon; however, a complete main clause generally should precede it.

Colons are also used in certain conventional situations such as to separate the hour from the minute when denoting time (12:30), after a salutation in a business letter (Dear Sir:), and between the chapter and verse in biblical publications (Mark 3: 12).