In Mauritius all rental cars have yellow licence plates. Normal cars have black ones.
Sometimes, when you rent a car from a small company, they try to give you a car with a black plate because it is cheaper for them. In case you get controlled by the police or even are involved in an accident, this can cause you trouble. Especially, because the car might not be properly insured.
If you are searching for a reliable company, you might wanna check out our site:
http://www.maki-car-rental.com/rent-a-car/mauritius.html
Alternatively you can go to one of the car rental companies at the airport, as the are likely to play by the rules.