Okay, I'll make this short and sweet because I'm sure it's been asked 8 thousand times before and I just haven't found the right thread for it.
It wasn't just asked "8 thousand times", it was also *answered* that much...for example here:
http://www.fsdreamteam.com/forum/index.php?topic=5728.msg60647#msg60647And of course, is explained in the manual too, page 19:
The airport handler is related to the ICAO code of the airport and the atc_parking_codes of the parking spot and it interacts with the ICAO specifications in the sim.cfg file of the vehicle itself.
This means, the airplane repaint YOU are flying, doesn't have ANY effect on the ground handler, and this is of course correct because, imagine if you fly internationally, it would be silly if a United airplane would be served by a United crew in Japan... The atc_parking_codes of the airplane you fly, ONLY affects the choice of ULD Pallets (if your airplane loads pallets, of course) and this is also correct and made intentionally, in order not to have pallets on arrival different than pallets on departure. If your airplane doesn't have an atc_parking_codes, pallets will be in the ground operator livery instead.
To be serviced by a specific operator, like United, you need to go to a United *parking* which is what you are supposed to do in the first place. But if that parking is flagged with multiple codes, the selection will be random between all candidates.
We are considering to add a feature to let the user have a selection menu to choose the ground operator, when there are many candidates, but it's not there yet so, the program right now is working as is supposed to do.
If you download the PaintKit, the documentation that comes with it explains in greater detail how the scoring system of the ground operators choice works.