Example 1
Scrooge McDuck has one major flaw in his personality: greed.
The colon in this sentence is used to make a pronouncement about Scrooge McDuck, specifically that he's greedy. We find it more troubling that he's a millionaire who doesn't own a pair of pants.
Example 2
Only five actors have won back-to-back Academy Awards: Tom Hanks, Spencer Tracy, Luise Rainer, Katharine Hepburn, and Jason Robards.
We don't know about you, but we are positively shocked that Meryl Streep isn't on this list. We're not surprised that this list is introduced with a colon, however, since it's preceded by an independent clause. And here's an insider tip for you. If you can replace the colon in a list with the word namely, then using a colon is appropriate.
Example 3
Tom has only one rule in life: never eat anything bigger than your head.
Not even a burrito?! Here, the colon sets the reader up for a surprise. The sentence essentially says, Tom has only one rule in life—wait for it—never eat anything bigger than your head.