For a long time there were two organisations responsible for the ambulances in Amsterdam, the GGD (Municipal Health Service) and VZA (United Medical Transport Amsterdam).
Since 2012 the Dutch government forbid the use of multiple ambulance services in one area, so the GGD and VZA merged into Ambulance Amsterdam, the organisation now responsible for all the ambulances in Amsterdam.
Because the ambulance care wasn't regulated on the nation level first, every region had it's own ambulances with their own style. Because of the aggregation of the 'gemeentepolitie' (Municipal police) and the 'rijkspolitie' (national police) in 1993, a national style, now know as BiZa/BZK striping, was designed for the police vehicles and, because of recognition, other emergency vehicles.
Most of the GGD ambulances adopted this new pattern. However, the VZA didn't like the new style and decided to keep their own, as it was better visible and thus safer to use. Somehow they managed to maintain their style until the merging of the GGD and VZA in 2012.
New ambulances are now only produces with the yellow color and BiZa/BZK striping. There are still a few white/orange VZA ambulances, but they will sadly disappear in the coming months.