What is a parts of speech colon?

Either, A. you mean the part of a persons body, which would mean colon would be a noun. You might, however, be talking about the symbol (:), which used as a Word would be a noun as well. However, a colon, when used in a sentence, is not a part of speech, it is a different category of grammar called punctuation. A colon is used to give a subtitle, to give the time of day, or to announce a date.

Examples:

The first examples (which is actually the heading) announces that I will give examples of colon usage.

Power Rangers: The Movie

In the second example, the colon is announcing that the program about the Power Rangers you are about to watch is the Power Rangers Movie.

Date: January 2nd, 2012

The third example shows that a colon can be used to announce what the date is.

11:30 P.M.

The fourth and final example shows that a colon goes between the hour and the minute in the time of day.