A colon after an independent clause signals that you're going to add further clarification or explanation. There are a few different grammatical elements that could come after a colon, but what comes before the colon is always the same: colons can only follow independent clauses.
You should always be able to identify a specific phrase in the first part of the sentence that the list defines or expands on. That phrase often builds suspense by suggesting that further information is about to be revealed. It is bolded in the examples on this page.
There are three grammatical elements that can follow a colon: a list, an emphatic appositive, and an independent clause.
I thought I had everything I needed for the camping trip: a tent, a sleeping bag, a backpack, and enough food for two days.
The following can come after a colon: a list, a definition, a clarification, or an explanation.
(See this page if you need to review appositives.) This is very similar to its role introducing a list, except that there is only one list item. In this situation, the colon emphasizes the word or phrase after the colon.
I mistakenly thought I only needed one item for my camping trip: a tent.
There was only one thing that could have ruined the trip: rain.
A colon can only follow one type of clause: an independent one.
The second part of the sentence expands on a specific phrase from the first part of the sentence: the one that is bolded in each example.
A colon can appear between two independent clauses, like a semicolon, if the second clause explains the first.
It was easy to identify where I went wrong: I should have brought a tarp.
Colons have an important purpose: they let you expand on or explain the first half of your sentence.
Colons are often misused when introducing lists. The following sentences are incorrect because there isn't a full independent clause before the colon.
Incorrect: Her favorite colors were: blue, red, and purple.
Correct (no colon): Her favorite colors were blue, red, and purple.
Correct (with colon): She had three favorite colors: blue, red, and purple.
Incorrect: For this assignment, you need to: start with a header, write 4 paragraphs, and include a works cited page.
Correct (no colon): For this assignment, you need to start with a header, write 4 paragraphs, and include a works cited page.
Correct (colon): For this assignment, you need to do the following: start with a header, write 4 paragraphs, and include a works cited page.
Another common error is trying to extend the sentence too far past the colon. The sentence needs to end right after the list, appositive, or explanatory clause.
Incorrect: I learned to cook three dishes in the microwave: pasta, scrambled eggs, and brownies, before I started using the stove.
Correct (no colon): I learned to cook three dishes in the microwave—pasta, scrambled eggs, and brownies—before I started using the stove.
Correct (colon): Before I started using the stove, I learned to cook three dishes in the microwave: pasta, scrambled eggs, and brownies.