I was wondering whether using a colon is incorrect if I used it as follows:
I’m sorry: I only saw your message this morning.
My logic for using the colon rather than the semicolon is that the relation between the apology and the succeeding statement is direct. The reason why I am sorry is as I didn’t see the message until the morning.
This use of the colon to show a more direct relationship between the first clause and the second is really emphasised in the University of Sussex’s English grammar guide. For example, the author writes that:
Lisa is upset; Gus is having a nervous breakdown.
The semicolon now suggests that the two statements are related in some way. The likeliest inference is that the cause of Lisa's annoyance and the cause of Gus's nervous breakdown are the same. Perhaps, for example, both are being disturbed by building noise next door. (Remember, a semicolon connects two sentences which are related.) Now try it with a colon:
Lisa is upset: Gus is having a nervous breakdown.
This time the colon shows explicitly that Gus's nervous breakdown is the reason for Lisa's distress: Lisa is upset because Gus is having a nervous breakdown.
This is the basis of my logic. I don’t know if when one is unsure, it is better to err on the side of the semicolon, as ‘who’ is suggested, for example, with who vs whom.
It seems whenever I look up ‘I’m sorry’, only variants with a comma or semicolon are shown.