Dear J___ C___:

You wrote:

>>Is it ever correct to use a comma preceding a list (series) or must it always be a colon?

>>John and Mary had three children, Ronnie, Jackie, and Joy. OR

>>John and Mary had three children: Ronnie, Jackie, and Joy

If you want to be technical about it, in this case people sometimes use a comma because the list is actually an appositive. That is true in most cases with lists like this–they really are appositives at the end of a sentence.

The problem with such lists is that because there are more than two items in the list, the list itself has commas. Since the list itself uses commas, it can be confusing to the reader to use another comma to set off the list. Using a colon makes it cleaner looking and easier to understand. The purpose of punctuation, after all, is to help us understand what we read. Stick with the colon.

If there were just two items in the list so that you did not need to use any commas, then a comma separating the appositive would work and be clear enough, though a colon is still OK because it is setting off an appositive at the end of a sentence.

John and Mary had two children, Ronnie and Joy. OR
John and Mary had two children: Ronnie and Joy.

I hope this helps.