The colon (:) is used less frequently than most other punctuation marks. It is used to introduce both lists and examples related to the preceding sentence. For example,
The following are important considerations when hillwalking: the weather forecast, the length of available daylight, and the steepness of the terrain.
The family had never been poorer: their cupboards were empty, their bank accounts were overdrawn, and they were facing eviction from their home.
A colon can be used to amplify what comes before it. For example,
The job was finished: after five long years of waiting, the house was ready.
Similarly, it can be used for emphasis. For example,
There was really only one city he wanted to live in: London"
A colon can also be used to introduce a quotation as long as the quotation is a complete sentence. For example,
His reference email finished with my favourite bit: "He was worse than being a man short."
Colons are often in titles with two related parts. For example,
The course was titled "A Passion For Profit: Making Your QMS Work For You."
When determining whether to use a colon, it can sometimes be helpful to think of it as an equal sign as used in mathematical equations, where the colon sits between different expressions that essentially say the same thing. For example,
My daily breakfast meal has only three ingredients: Kale, peppers, and mushrooms.
My daily breakfast meal ingredients = Kale, peppers, and mushrooms.