LinguaBeasts
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Colon Cole, a yellow punk pest in round dark glasses and a black studded jacket, holding up a colon beside a crate labeled “Cole builds bridges. Don’ leave it hanging.”

Colons

Colon Cole

“Cole builds bridges. Don't leave it hanging.”

What's the problem?

In running prose, our house style places a colon after a complete introductory clause. That complete clause is necessary but not sufficient: what follows the colon must explain, specify or illustrate what the clause sets up, such as a list, an elaboration or a quotation. What follows can be a list or a phrase as well as a full clause. Don't place a colon between a verb and its object or between a preposition and its object. Some scoped conventions are not errors: times (9:30), ratios (3:1), form labels (Email:), headings, salutations and vertical lists. Capitalization after a colon varies by style; our house style uses lowercase unless what follows is a proper noun, a full-sentence quotation, or two or more complete sentences. A colon can present a writer's explanation, but it doesn't prove that the explanation is true. In these exercises, each running-prose sentence uses at most one colon; that's an exercise convention, not a universal rule.

Context: A safety notice. The kit contains exactly three supplied items, and the writer supplies the broken projector as the reason the room has changed.

Why it works
"The kit includes" isn't complete without its objects, so no colon goes there. "The room has changed" is complete, and what follows explains it, so a colon fits. A period or semicolon could also join that second pair.