Here is a summary of rules for using the colon:
Introducing a list of information
|There are three primary colours: red, green, and blue.|
Leading to a conclusion or point of emphasis
|I'm going to paint my bedroom in my favourite colour: blue.|
Making a point of clarification
|I told them over and over again what my favourite colour was: I said it was blue.|
A colon is used for one of three things:
As you can see above, there should be no space before a colon. It is a common mistake to put a space before a colon, but there is no need or reason for this. A colon is immediately associated with the word before it.
CORRECT A colon is used when you want to:
INCORRECT A colon is used when you want to :
The above examples are using the colon to introduce a (bullet point) list, but that is not the usage you are most likely to see. Even if you see a bullet point list, you might not see a colon used at all, as the options for finishing the list might be thought of as fragments of the opening sentence. See here for Bullet Points.
The colon is used to introduce or 'point' to something that follows a statement, and we can make conclusions about that statement or clarify some relevant details by using a colon and then attaching those details to complete the sentence. Here are some examples:
There are three primary colours: red, green, and blue.
In this example, the first part of the sentence could easily end at 'colours'. However, the writer wants to expand upon the first part by providing the details which will ensure that people don't ask for them. So, you might think of this in terms of a question being asked and answered at the same time. Individually, the question and answer would look like this:
Q: What are the three primary colours?
A: Red, green, and blue.
The example above is a concise way of writing this question and answer together in one sentence. Now look at the next example, in which a question is anticipated by using a colon and providing the answer.
I'm going to paint my bedroom in my favourite colour: blue.
Again, the writer has ensured that the reader does not need to ask, "What is your favourite colour?", or "What colour will you paint your bedroom?". The answer is provided by attaching it in a very concise way, using a colon to indicate that it is coming next.
I told them over and over again what my favourite colour was: I said it was blue.
As you can see, the statement following the colon is an independent clause (subject + verb) that has been associated with the opening statement. In this way, the reader again doesn't need to ask the question "What did you tell them?", because the unspoken question has been answered.