Wow some weird information in here.
The Green frequency isn't always the one you should call. That is the class G frequency so if you are flying in class G then yes, that would be the one you would call. Be careful using blanket statements.
Otherwise Ian is right. You need to speak to your instructor to find out who is the most appropriate to call. Basically as far as ATC is concerned you call first sector for a clearance that you will be entering CTA. However since DAH is a bit dry I suspect you won't necessarily know which sectors are which.
For example the 119.5 is actually an approach sector (Sunshine) that sets the aircraft up for the CTA approach sector (BAC - Brisbane Approach Cooloongatta) which is 123.5. Yes it is confusing hence why you should always check ERSA, check any VFRG's and last but not least, if you really don't know, call ATC on the current frequency you should be on and ask us. We are not that bad. I regularly get guys as far south as Coffs calling and asking for their Gold Coast squawk code so they can get it on the txpdr early. No problem with this also.
Pearo and Rage are also right in that last post. However don't only check out the towers, come in to Brisbane or Melbourne center. If you want you should be able to get in and see how we work and maybe plug in behind one of the controllers and have a listen. If you are worried, one of the ATC's on here (I'm happy to oblige) might be able to help organise this for you. Alternatively there are regular pilot nights held in the center here that include a trip into "the room". These are for VFR and PIFR pilots more than the RPT's so make use of them.
EDIT: Aplund or whoever was saying that Approach were dicks, remember these guys have the stressful jobs and are under high workloads, you may not be able to hear it so perhaps give them a break...