A colon, like the comma and the semicolon, is a joiner.
Think of it as a kind of connective gesture towards what's coming next. If you were reading a sentence out loud, when you got to the colon you might pause and raise your hand in front of you, ready to count off what comes next on your fingers because what comes next is, more often than not, a list.
For example, you might say, "I have three favorite television shows: Parks and Recreation, Friday Night Lights, and Top Gear." But here's the kicker: for a number of reasons, you'd never say, "My three favorite television shows are: CSI, NCIS, and SVU." For one thing, you'd be wrong about that. But more importantly, a colon can only follow an independent clause, and no matter which way you say it, "My three favorite television shows are" simply isn't one.
A colon doesn't always have to precede a list, though. For example, you would be right to write, "Facing the rapacious zombie, Deputy Grimes had a choice: kill or be killed." Of course, you would be wrong to write, "Facing the rapacious zombie, Deputy Grimes's only choice was: kill or be killed." This colon would break the full sentence rule, and you should just chuck it entirely.
The important thing to remember here is that a colon must connect a full sentence to something that is directly related to it, whether it's an explanatory list or clarifying statement. You'll notice that, as a reader, you almost always have an idea of what's coming after the colon because it explains, expands, or elaborates on the complete sentence that came before. It's as if there were a little narrator inside the sentence, waving his hands and screaming, "Now I'm going to tell you what Deputy Grimes's choice was!" It gives the reader a little breather before the sentence launches into its elaboration.
We like to think of the colon as kind of a wait for it… . Just with a little less personality.