Which of the following colon rules is correct?

Always capitalize the word following a colon in a list.
Include a colon to emphasize important information.
Place a colon after greetings in any type of letter or correspondence.
Use a colon to separate two sentences.

The correct rule is: Use a colon to separate two sentences.

The correct colon rule among the options given is:

- Use a colon to separate two sentences.

A colon is used to introduce a list, emphasize important information, or separate clauses in which the second clause explains or elaborates on the first. When using a colon to separate two sentences, the first part should be able to stand alone as a complete sentence, and the second part should directly relate to or expand upon the first part.