Colon Rule: Use a Colon to Introduce a List

The sentence preceding the colon should be a complete independent clause.

Correct
I dislike birds as they posses unsavory qualities: ugliness, loudness, and a tendency to stand by windows coughing at tired people.
A proper eggplant has three characteristics: it is purple, oblong, and not the progeny of birds.

These examples are correct because the sentence before the colon is not dependent upon the list; the list emphasizes or expands upon the ideas introduced by the sentence before it. 

Just look at it; it’s probably looking for some exhausted youth to spit at.

Incorrect
Birds: ugly, loud, and cough too much.
The proper eggplant is: purple, oblong, and of no relation to birds.

The clause preceding the comma is not a complete sentence so it relies upon the colon to be a complete idea, and that is incorrect.