THe important thing to remember with colons is that the portion of the sentence that comes BEFORE it must be an independent clause.
For example, we can say:
I have three favorite subjects in school: math, chemistry, and language arts.
We cannot, however, say:
My three favorite subjects in school are: math, chemistry, and language arts.
There are several roles that the part following the colon to play, but pay closest attention to the part preceding it to make sure it can stand on its own.