Japanese license plates come in three main types (there are sub-types). The white is for what is considered a 'regular' size car (which would be a compact car in America), the yellow is for a compact car (a sub-compact car in the USA) and the green license plate (not shown) is for a commercial vehicle such as a taxi. The classification means that the cars are taxed differently. The smaller the car, the less tax is applied to it across the board, including toll fees for bridge and highway use. The Japanese will even get out of their cars at the toll collection points to debate their tax fee, all done politely.
Vehicle Registration Plates of Japan Wikipedia Entry