In Japan, the color schemes used by license plates are standardized at a national level. Plates with black text on a yellow background are used for "light vehicles" (sometimes glossed in English as "kei car" from 軽 kei "light"). For example:
A three- or four-wheeled vehicle is deemed a "light vehicle" if its engine displacement is 660 cc or less and meets a few size and weight restrictions.
I don't know much about cars, but it seems that 660 cc is quite small. The five top-selling cars in the U.S. as of January 2016 were the Toyota Camry (2500+ cc), Honda Civic (1500+ cc), Toyota Corolla (1800+ cc), Nissan Altima (2500+ cc), and Honda Accord (2400+ cc). And these are all reasonably-sized sedans, not Hummer-esque monstrosities (6000+ cc).
These cars are fairly popular in Japan (as of Mar 2015, 39% of all vehicles in Japan were light vehicles; this is up from 26% in 1998), for reasons apparently having to do with how Japan taxes and regulates vehicles. They also sell reasonably well in South/Southeast Asia.
I would agree with Nori that light vehicles are conservative, primarily because they're often rather boxy and unattractive (because of the size restrictions) and because they accelerate slowly and generally have poor "performance" (because of the small engine). They seem to have many benefits, especially in urban environments (easier parking and maneuvering; lower taxes and cheaper insurance; better gas mileage; and apparently there's less bureaucracy involved in registering one). But you're not going to impress almost anyone with a light vehicle.
(Semi-related plug: if the minutiae of Japanese motorcycles interests you, check out Bakuon!!, which is basically Top Gear, except with cute girls riding cute motorcycles.)
