Video Courtesy of (site-affiliate) Kaplan GRE prep.
RULES FOR COLONS
1. Use a colon when making a list, when what precedes the list is an independent clause.
CORRECT: There are four ingredients necessary for a good party: music, lighting, food, and personality.
There are four ingredients can stand alone, so the colon separates it from the list.
Do not use a colon to separate a preposition from its objects or a verb from its complements, since the clause will not be independent.
INCORRECT: My ancestors came from: Poland, Russia, and Ukraine.
The colon incorrectly separates the preposition from its objects, and leaves My ancestors came from, which is not an independent clause. Instead, the sentence should read:
CORRECT: My ancestors came from Poland, Russia, and Ukraine.
INCORRECT: The boys ran home and ate: cake, cookies, and soda.
The colon incorrectly separates the verb ate from its complements. The sentence should read:
CORRECT: The boys ran home and ate cake, cookies, and soda.
2. Use a colon after a complete statement to introduce related ideas:
CORRECT: The coffee shop is the best on the block: it has great scones, a full menu, and a great atmosphere.
If, when you are writing your essay, you are in doubt about whether or not you may use a colon, it is best to play it safe and separate your statements with a semi-colon or a period.