Colon
The colon is used after an introductory statement when the following statement explains, reaffirms, elaborates, elucidates, illustrates, or summarizes the preceding statement. Everything that follows a colon should be directly related to the topic in the introductory clause. The first word following a colon, even if it is the beginning of a complete sentence, should not be capitalized unless it is a proper noun. For stylistic reasons, it might be capitalized, but only do so for a good and defensible reason (especially to your editor). There is no space before a colon. You may find either one or two spaces after a colon. As with spacing after a period, it is becoming more common to see only one space after a colon.

Usage
1. Time

Example: 9:30 a.m.

2. Business salutation

Example: Dear Sir or Madam:

3. Headings

Example: Truck for sale: Dark blue, automatic, short box.

4. Title: subtitle

Example: Hiking in Glacier National Park: A Guide for Beginning to Advanced Hikers

5. Introduction of a series or list

Example: Beth painted her bedroom three colors: purple, pink, and blue.

5A. Don’t use a colon if one or more of the items in the list are needed to complete the introductory statement.

5B. Use a colon after the terms “as follows” and “the following”

5C. Don’t use a colon if a list is introduced with a phrase such as “such as”

6. A colon points the way to a revelation (the colon adds emphasis to the final clause)

Example: Beth put on her gloves, slid the envelope of pictures in her purse, and picked up the rope: it was time to complete her revenge.

7. A colon points to an explanation. (Answers the question “why?”)

Example: Billy stared at the wall: he couldn’t look her in the eyes after what he had done.

7A. Answers the question “what?”

Example: I have a secret: I robbed the bank.

8. A colon can point to an elaboration

Example: He was cautious with the people he made business deals with: he hired a private investigator to look into their backgrounds before agreeing to anything.