See "D-a-s-h-e-s" [pdf 512kB]
To prepare for a list, a restatement, an amplification, or a dramatic
shift in tone or thought:
- Along the wall are the bulk liquids—sesame oil, honey,
safflower oil, and that half-liquid, “peanuts only” peanut
- In this last semester, Peter tried to pay more attention to his
priorities—applying to graduate school and getting financial
- Everywhere we looked there were little kids—a box of Cracker
Jacks in one hand and mommy or daddy’s sleeve in the other.
- Kiere took a few steps back, came running full speed, kicked a
mighty kick—and missed the ball.
In the first 2 examples, the writer could also use a colon. The
colon is more formal than the dash and not quite as dramatic.
This is a case of both list and amplification. The phrase "blue, red, and green" lists and amplifies on "three cars."