As for Japan, they have a large range of vehicle license plate classes. The Yellow plates were common -- smallest engine size (<660 cc) that was authorized for highway travel. They are normally required to be in the "slow" lane. There are roads in the mountain areas that prohibit these cars due to their weak engines.
The US Military for the most part have "Y" plates (white) or "A" plates (yellow). Many of the smaller engine vehicles are used exclusively on base.