Explanation:

White number plate
Cars bearing black lettering on a white background are private vehicles. These are used solely for private purposes and can't be used for any commercial purpose. 
Yellow number plate
Commercial vehicles such as trucks and taxis have a yellow number plate with black text. These vehicles can be used for any commercial purposes. However, the driver of the vehicle must have a commercial driving permit.
Black number plate with yellow lettering
These are commercial vehicles available on rent for self-drive. These cars can be put to any commercial purpose without the limitation of the driver having to own a commercial driving permit. 
Light blue number plate
Vehicles belonging to foreign consulates bear white lettering on a light blue background. These number plates bear UN, CD or CC, which stand for United Nations, Diplomatic Corps and Consular Corps respectively.