Use a colon to introduce an explanation, an example, an appositive, a series, a list, or a quotation.
At the baby’s one-month birthday party, Ah Po gave him the Four Valuable Things: ink, inkslab, paper, and brush.
—Maxine Hong Kingston, China Men
Use a colon rather than a comma to introduce a quotation when the lead-in is a complete sentence on its own.
The 2013 State of the Union address ended with a bold challenge: “Well into our third century as a nation, it remains the task of us all…to be the authors of the next great chapter in our American story.”
Colons are also used after salutations in formal letters; with numbers indicating hours, minutes, and seconds; with ratios; with biblical chapters and verses; with titles and subtitles; and in bibliographic entries.
Dear Dr. Ito:
4:59 pm
a ratio of 5:1
Ecclesiastes 3:1
The Joy of Insight: Passions of a Physicist
Boston: Bedford/St. Martin’s, 2013