Most writers do not hesitate to use, and often misuse, the comma; to a lesser degree the semi-colon finds its way into our compositions. But the colon rarely appears. Is it because we fear the uncommon?
We do use it routinely to separate volume and page in referring to a publication (Volume 9:23) and in time references (2:45 pm); but we seem reticent to use it in daily writings such as letters. Too bad, because used properly it can add color to your prose.
The colon is used to fulfill an introductory but incomplete thought. We all knew what to expect of the shortstop: great glove, good arm, but no bat. We knew the moment the cat saw the dog: when its hair stood on end and it hissed. Note that in each illustration what follows the colon is not a complete sentence and therefore does not commence with a capital letter, unless of course the first word would be a proper noun.
The police officer startled them with his options: Either they must reveal the suspect’s whereabouts or face arrest themselves. Note here that a full sentence follows the colon and begins with a capital.
In each of the illustrations a dash might have been used instead of the colon, but some of the formality and impact would have been lost. In the last illustration a semi-colon might be technically proper, but it would tend to blur the effect. As in much of grammar, subjectivity rules. It is all about injecting life into your prose: step right up, employ the colon (properly), and reward your reader.
— Ken Butera