New Cars and the "Green Label"
Strictly speaking, the "Green Label" is intended to let the consumer know about the environmental impact that a particular car will have by detailing the amount of Carbon Dioxide (CO2) the car will produce per kilometre. Luckily, these CO2 figures are also a pretty good indication of how fuel efficient a car is - low CO2 levels imply that a car that is very efficient. These bands are consistent with the new road tax prices too, so you'll be able to see how much tax you'll be paying too - check out the Emissions & Tax page for more information.
To find out what band you car is in, have a look at our What is my MPG? page which will give you information about emissions and tax as well as fuel consumption.
The label also contains some other useful information like the model's official fuel consumption figures in MPG and Litres/100KM - in particular it shows the amount of road tax the vehicle will incur as well as an estimation of the fuel costs for a distance of 12,000 miles. This figure is based on an annually calculated average fuel cost so it wont give you an exact figure, but its a good indication.
The manufacturer needs to provide this information - it can either be with the car on display, or available elsewhere at the dealership. If you don't see it, ask for it and the dealership will have to give you the information.