You talked about separating independent clauses with semicolons, but when would you use a colon to separate independent clauses instead? Are the uses completely different, or does the author get a choice?
That’s an interesting question. When a semicolon joins two independent clauses, it’s basically saying, “Here are two equally important bits of information that are part of the same thought.” It’s being a little coy about the exact relationship between those clauses. Sometimes you can replace it with a comma and a conjunction, but in that case it’s up to the reader to infer what the conjunction would be. Sometimes, as in the final example I gave in my semicolon post—”Happiness isn’t something you experience; it’s something you remember”—there’s no way to replace the semicolon with any linking words while preserving the sense and meaning.
A colon between two independent clauses, on the other hand, can usually be replaced with something like “that is” or “because” with no real harm done. That’s because the colon, when used like this, is meant to explain, restate, or illustrate what came before, which means you usually have the option of turning it into a subordinate clause.
Here’s a couple of examples:
The insurance company declared the car totaled: Repairs would cost more than the car was worth.
The insurance company declared the car totaled, that is, repairs would cost more than the car was worth.
The dodo was utterly alone: It was the last of its kind.
The dodo was utterly alone because it was the last of its kind.
You can’t really use a semicolon in these sentences because you lose what the colon is signaling: that the second phrase is clarifying the first in some way. Two independent clauses linked by a semicolon could be related in a number of ways, but the colon is much more specific and therefore not usually interchangeable.
However! Every so often you might run into a sentence where either could work:
A famished Johnny tucked into the long pork eagerly: He assumed the phrase referred to some new breed of pig.
A famished Johnny tucked into the long pork eagerly; he assumed the phrase referred to some new breed of pig.
I think the colon is the best option here, but if this sentence came across my desk with the semicolon, I’m not sure I’d change it.
If you think either could work but you’re genuinely not sure which is best, you may be able to avoid the question altogether with a handy-dandy em dash.
Bonus advice: If the phrase after the colon could stand as a complete sentence on its own, many American sources recommend capitalizing the first letter after the colon. If the phrase that follows the colon is not an independent clause, however, the first letter should always be lowercase.