Colon

A colon may be used to introduce a formal question, quotation, amplification, example, or list. In essence, it replaces that is, for example, such as, namely, or for instance and therefore should not be used with any of these words:

The study involved three metals: chromium, molybdenum, and titanium.

A colon should not be placed between a verb or preposition and its direct object:

The three metals under study were chromium, molybdenum, and titanium.

Comma

The comma denotes a slight pause. Effective use involves good judgment. Ease of reading is the overall goal.

The rules below are well accepted, and you can safely use them as a guide:

Use the serial comma:

Penn is a leader in developing efficient, green, and inexpensive modes of transportation.

Do not use commas before or after Inc., Ltd., Jr., Sr., and the like:

“I didn’t know what that meant,” said John Doe Jr.
John Doe Jr. decided to attend Penn.

Use commas before and after the word “too” in a sentence:

“You, too, can be a style fanatic,” said John Doe Jr.

Use a comma when beginning a sentence with an adverb:

Truly, we are making history together.
Ideally, we should set aside funds against future income shortfalls.

Use a comma before a conjunction that joins two independent clauses:

Thank you for your donation to the Penn Fund, and welcome to the Benjamin Franklin Society.

Use commas around nonrestrictive (nonessential) phrases or clauses. Do not use commas with a phrase or clause that is restrictive (essential to meaning of sentence):

The student-led initiative, which began in January, is the first of its kind in the Northeast. (nonrestrictive)
The professor who won the Nobel Prize is on the far right in the photograph. (restrictive)

Use a comma to introduce or set off a quotation unless the quoted word or phrase functions as the subject or object of the sentence. Do not use a comma when a quotation is immediately preceded by the conjunction “that.” Commas never should be used in combination with exclamation or question marks.

President Gutmann says, “Our students are the smartest on the planet.”
In her Commencement address, President Gutmann said that “our students are the smartest on the planet.”