Grammatically Correct: The writer's essential guide to punctuation ...

Colon(:)
The colon acts as a signal of anticipation, drawing the reader's
attention to what comes after it. (In the classic reference book
Modern English Usage, the grammarian H.W. Fowler describes it
as "delivering the goods that have been invoiced in the preceding
words.") Like the semicolon, in some cases it is required, and in
others it is used for effect. Its functions fall into the following main
categories:
• Introducing the text that follows
• Strengthening connections or adding emphasis
Writers are often unsure about the distinction between this mark
and the semicolon. A discussion of their differences and similarities
is presented at the end of this section.
INTRODUCING WHAT FOLLOWS
Use a colon when the first part of a sentence is an introduction, a
lead-in, or a buildup to what follows.
USE A COLON WHEN A SENTENCE
CONTAINS A "QUESTION/ANSWER"
A colon serves to cue readers that a sentence consists, in a sense,
of a question and an answer. That is, it conveys the signal that the
text preceding it has just raised an implicit question, to which the
remainder of the sentence is about to provide a response.
The situation was becoming desperate: Supplies were running low, and
winter would soon be setting in.
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