The colon has a rather unique function. It generally indicates that something is going to follow. It’s somewhat like an introduction to what comes next.
Writing practice will give you confidence working with this mark of punctuation.
There are several common usages:
(a) After the Salutation in a business letter
(b) After the introductory statement for a list (often including the words as follows)
The list of clients is as follows:
(c) Joining two sentences without a conjunction
rather rare occurrence is when this mark of punctuation joins two
sentences without a conjunction. This works only if the second sentence
is seen and understood to be a consequence or result or explanation of
the first sentence - and is understood that it follows clearly.
Something always follows this punctuation mark. When you see it, you realize that there is an explanation to follow – something always follows.
You expect to read something else immediately following.
Genevieve is travelling by bus:
The sentence above cannot be complete - so what follows? A logical explanation?
Genevieve is travelling by bus: her car is being repaired.
In the above completed sentence, note that both parts can be complete sentences. Look at the second part. The second part explains or completes the first sentence.
The colon indicates that there is something (an explanation) to follow.
(d) Writing a ratio
The odds are 5:1 in favour of a Yes vote.
(e) Writing times:
The meeting began on time at 10:00 a.m.
The meeting adjourned precisely at 4:00 p.m.
Avoid this error:
Do not add punctuation in a sentence between the verb ("need") and the object ("lettuce").
For this salad, you need: lettuce, peppers, and onions. (incorrect)
For this salad, you need lettuce, peppers, and onions. (correct)
For this salad, you need the following:
Other examples of good format:
Dear Mrs. Smith: Thank you for your letter of ...
My shopping list is as follows:
Punctuation has an important function to play in making our writing clear. Punctuation allows us to group words and phrases together in a meaningful way.
Without punctuation, text would be chaotic and without meaning.
When compared to other marks of punctuation, this one
has a limited number of correct uses, so it should be relatively
straightforward to learn to use this mark of punctuation correctly.
LINK - Return from Colon to Professional Business Writing