The colon is a rare punctuation mark in this day and age, but when you need one, you need one.
The most common use of the colon is for listing data.
Page size: 5.13 x 8
(Incidentally, these are the settings I use when I want to make a proof copy of my book.)
Colons can also be used in a sentence to organize a list composed of words, rather than numbers.
Isaac had, for the most part, been fairly tidy, but here and there, something was out of place: a magazine casually tossed on the sofa; a post-it note stuck to his computer monitor; a sock half hidden under a chair.
The list of items following the colon detail the things which are out of place.
Colons can also highlight a single item; the list doesn’t have to be two or more things.
Besides, you forgot my exception to the rule: I will do anything for the benefit of others.
If Joshua had more than one exception to his rule, the sentence would still be structured the same.
Besides, you forgot my exceptions to the rule: I will do anything for the benefit of others, and I will break any rule of mortality if I feel I’m doing an even greater good as a result.
A clause or sentence following a colon can explain the preceding sentence.
“You and I are exactly the opposite: we trust Ciaran because he’s trustworthy, and we don’t trust Jonas because he’s not.”
What’s the situation? Kalyn and Anselm have a view which opposes everyone else’s. Why? Because they know Ciaran is trustworthy.
“And that’s why the vast majority of us are pretty strict about our morality—despite the fact we can bend people’s wills to our own: we don’t want anything else on our souls to have to answer for.”
Here, the situation is that the vampires are strict about their morality. Why? Because they’re afraid of the spiritual consequences.
Colons can also introduce a question or statement.
“Answer me this: do you think your mother died of a broken heart?”
“Give Anselm a message from me: he is to stop this subservient nonsense.”