Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
The next witness testified that she had been fired from her job as a line
supervisor. Her offense: trying to organize the factory's 200 workers,
who earned an average of $90 a week.
The text in both cases is basically the same, but the message
is subtly different. A contrast exists between the reference to an
"offense," which implies some inappropriate behavior, and the actual
actions of the subject, which seem to be laudable. Assuming the
writer is trying to portray the subject's actions in a sympathetic or
supportive light, version B achieves this better. In version A, a reader
who wasn't paying close attention might get the impression that the
writer as well as the factory owners viewed these actions as a genuine
offense. In version B, the colon heightens the contrast, thereby
making it clear that the word offense is being used ironically.
Consider the two versions of the next example:
Wife assault cannot be dismissed as merely a symptom of marital
problems. It is a criminal activity that is a manifestation of the abuser's
inability to deal with frustration and anger.
Wife assault cannot be dismissed as merely a symptom of marital
problems: It is a criminal activity that is a manifestation of the abuser's
inability to deal with frustration and anger.
Here, version A has less impact than version B, since the contrast
between the key words-the relatively benign "marital problems"
versus "criminal activity"-is diminished by putting them in separate
sentences.
If the goal is to run these two sentences together, would not a
semicolon or a dash do as well? The answer is no, for somewhat
subtle reasons. The first part of this sentence is acting as a "buildup"
to the second, in that it tells the reader what wife assault is not.
The colon then conveys the message that the remainder of the
sentence will set you straight as to what it is. (Put in the terms
given earlier, the implied question is, Well, if wife assault isn't a
marital problem, what is it?, and the answer is, A criminal activity.)
Neither the semicolon nor the dash quite performs this function.
The semicolon does not act to draw the reader's attention to what
follows it, and while the dash does, it does so for purposes other
than answering a "question" raised in the first part.
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