There are half a dozen special uses for the colon.
1. Numerical expressions of time.
Example: 5:31 p.m.The colon goes between the hour and minute. If seconds are noted, a colon goes between the minute and second.
Example: He ran the marathon in 2:14:33.2.
(Two hours, fourteen minutes, and thirty-three point two seconds.)
Example: He ran the mile in 4:12.
(Four minutes and twelve seconds)
2. Periodical references in a bibliography or formal reference.
This may vary slightly depending on the form followed. Most frequently the reference is Volume:Issue Number or Volume:Page Number.
3. Bible references, Chapter:Verse.
Example: John 3:16
("The book of John, chapter 3, verse 16.")
4. Subtitles for books, periodicals, and articles are preceded by a colon.
Example: Ben-Hur: A Tale of the Christ
See also Capitalizing Titles of Things.
5. Salutations in business letters are followed by a colon.
Dear Ms. Hathaway:
See also Capitalizing in Letters, Commas in Letters, and the "Letter Writing" heading in the Table of Contents.
6. Colons follow labels that identify important ideas meant to get attention.
Warning: To be opened by authorized personnel only.
Notice: Do not use before October 15.
This is actually the rule that Grammar Slammer follows for displaying "Correct," "Incorrect," and the like for pointing out its examples.