Input Mask - quick question

Sorry I'm just an occasional Access user but need to tweak an existing
form in a database. This has a couple of fields for holding latitude
and longitude values, which we need to store as text values in
deg:min:sec format, eg 12:34:56 .

(When I say 'store' I mean that literally. The database file is
accessed by a completely separate custom VB program that reads these
two fields as strings and expects to see two colon characters in each
field string in the appropriate places.)

I'd been hoping to use an input mask so that users don't forget to
enter the colons, which they're apt to do. In other words, so that all
the user needs to do is to enter only the digits, with the colons
effectively being entered automatically

For the input mask I've tried using 00:00:00 and then 00\:00\:00
(wondering if the colons needed to be a literal) but in neither case
do the colons seem to get stored in the relevant database fields.

Anyone able to advise please?