It is conversational rather than colloquial. In any case that means it is not for written texts.
Also, I think donques' suggestion "As a rider to this" would only fit if the à propos modified the original point.
I've actually had been registered under another user some time ago, but I couldn't remember it and I've just come up with this one Anyway, I wasn't too active, but I hope now I'll be, as I am trying quite hard to study French
donques had provided : "While were on the subject" without the apostrophe in "we're" which should be included there as it stands for the "a" of "are". This might make the translation expression unclear to some Forum users.
Precision : "while we're on the subject, ..." (= "while we are on the subject, ...")
indeed a manner of saying "à ce propos, ..."