Sorry if I'm repeating myself but is there an easy way to configure beforehand what catering livery should be used for a specific airport so I don't have to pick one via the menu in real time?
Reading the PaintKit documentation, will explain how the scoring system works, so you might try to modify the catering SIM.CFG files according to what you learned.
However, any changes you made to the default SIM.CFG files that ships with GSX, will likely overwritten during updates, so it's probably safer if you do it only for your own custom repaints.