Alist should be introduced with a colon when the list is preceded by
a. a strong opinion
b. a complete thought
c. a dependent clause
d. a descriptive phrase

B. A complete thought

A colon is used to introduce an item or a series of items (a list), and it is usually preceded by a complete thought, therefore they are not preceded by a verb or proposition.

Examples:

Correct: I painted my room with a variety of colors: blue, light green, white, pink and yellow.Correct: Venezuela is divided into 8 regions: Andean, Capital, Central, Central-Western, Guayana, Insular, Llanos, Eastern and Zulian.Incorrect: I want: 4 fiction book, a pencil and a scissor.Correct: Here is what I want: 4 fiction book, a pencil and a scissor. "Here is what I want" is a complete thought.Incorrect: We're all going to your house including: Alicia, Sofia and Mark. Correct: We're all going to your house including Alicia, Sofia, and Mark. (Including is a preposition, therefore the use of a colon is wrong).