very endearing/sweet(if I'm allowed to use the term ;-) of the Colonel to speak about the subject of flannel waistcoats.

I have often tried to imagine the scene where he would mention such a thing.
Mrs Jennings shouting "Brandon! How's your aching shoulder?" and Brandon with no feelings of self consciousness (unlike Edward when asked upon to read) answers, "Thank you for your concern Mrs Jennings. I have started wearing flannel waistcoats and they offer really good protection,"

All this within Marianne's hearing. :-D

Which of course means that Marianne must know the context as well.
Of course later in the book the real reason might just come up. We have to wait and see.