Use the colon to introduce a list or a series:
The committee's study focused on the most critical areas: development of software, needed changes in computer systems, and recruitment of new engineers.
Use of namely, that is, etc.
If the list or series is introduced by such expressions as namely, for instance, for example, or that is, do not use a colon unless the series consists of one or more grammatically complete clauses:
The committee's study focused on the most critical areas, namely, development of software, needed changes in computer systems, and recruitment of new engineers.
We face several obstacles, that is: The employee must complain to the committee; the committee must review the complaint; and the commissioner must then decide the outcome of the dispute.
List Completes the Sentence
Do not use a colon to introduce a list that is a complement or object of an element in the introductory statement:
The agency must (1) publish the notice in the Federal Register, (2) wait the prescribed period of time, and (3) consider any comments received.
The committee rejected (1) the employee's evidence, (2) the supervisor's report, and (3) both parties' requests for relief.
The Expression as follows
The term as follows or the following requires a colon if followed directly by the illustrating or enumerated items or if the introducing clause is incomplete without the items:
The factors are as follows: (1) reasonable expectations, (2) intent, and (3) sufficient evidence.