A colon only serves to link cause and effect, rule and examples, and related forms of “X in some way implies Y” such as:
“The SDMB has three female Admins.: TubaDiva, Lynn Bodoni, and Gaudere” or “The saddest words of tongue or pen / Are surely these: ‘It might have been’.”
I don’t believe there is even an off-the-wall hypothetical situation where a colon and a semicolon may replace each other in the punctuation of a ‘proper’ English sentence.
What you have there is, as has been pointed out, a comma splice: a comma is being used to join two independent clauses. In formal writing, you should use a colon, a semicolon, or sentence-terminal punctuation (e.g., a period), depending on what exact effect or meaning you’re going for. In informal writing, you can pretty do whatever you damn well please.
People don’t speak in **any **punctuation. Punctuation, like all written language, is an external, arbitrary construction used to replicate natural, spoken language. To say that people don’t speak in semicolons makes about as much sense as saying people don’t walk in stop signs.
There are plenty of sentences where one could choose to use either a semicolon or a colon to give slightly different connotations. To use the OP’s example:
Stop picking at that: it’ll just get worse.
The second half of the sentence is an explanation of or justification for the first half.
Stop picking at that; it’ll just get worse.
The second half of the sentence is a continuation or elaboration of the first half.
I knew that someone would disprove me with the same degree of certitude that I know what comment will inevitably punctuate a question about Jacques Piccard’s 1960 descent to the floor of the Challenger Deep.* But as you note there are slightly different connotations resulting from the choice of punctuation. Perhaps I should have said that there are no instances in which the two marks are interchangeable without change of meaning.
Perfectly willing to agree with you there. A colon and a semicolon are not interchangeable in the sense that they’re render two sentences with identical meaning; however, the general thrust will be the same, even if the nuances change.
Only once. In 2010.
The simple rule is that you can use a semicolon anywhere you could use a period, and you can use a dash anywhere you could use a comma. The sense of the meaning will change, but they’re acceptable in the same constructions.
It’s also sometimes acceptable to use a semicolon in place of a comma, to delimit a list where the individual elements of the list contain commas. For instance, I might say “I have relatives in Los Angeles; Buffalo, New York; Athens, Ohio; and Paris.”. The meaning of the sentence would be unclear if I instead said “I have relatives in Los Angeles, Buffalo, New York, Athens, Ohio, and Paris.”.