I doubt it has anything to do with marketing because Abu Dhabi and Dubai Royal govt. aircraft do not wear Etihad or Emirates liveries, and Saudi Arabia have always kept their Royal fleet in SV livery since not so image concious times.
Kuwait govt. 744 infact does commercial flights as well depending availability.
Infact I had read soemwhere that these airlines actually operate and maintain the aircraft for the govt. using their codes etc. thats why the livery, as in they are part of the respective airliens fleet.