Colon, Comma, or No Punctuation After 'To Be' Before a Question
The Colon
Rulebase Summary
When a question follows a form of the verb 'to be' (is, was, are, etc.), three options exist depending on length, formality, and whether the question is direct or indirect:
(1) No punctuation if the question is short and runs smoothly into the sentence ('My question is why?'). (2) A colon for emphasis or formality ('My question is: Why?'). (3) A comma for a less formal feel ('My question is, Who did it?').
If the question is being restated from elsewhere in the transcript, a colon may introduce it with quotation marks. If a completing word appears between the verb and the question (e.g., 'My question is this:'), only a colon is correct.
Examples
Q. My question is, why did you wait three days to report the theft?
Q. My question is why did you wait three days to report the theft?
Short question flows naturally — no punctuation after 'is.'
Q. The bottom line is. Did he threaten you?
Q. The bottom line is: Did he threaten you?
Colon after 'is' for emphasis before a formal question.
Style Guide References (2)
Morson's
BGGP