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.
5. Salutations in business letters are followed by a colon.
Example: Dear Sir:
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.