Three of the country’s main conurbations – Berlin, Cologne and Hanover – have introduced new laws stipulating that the stickers must be displayed.
They reveal what levels of nitrogen dioxide (NO2) and particulate matter (PM) a vehicle emits. Red stickers indicate the highest levels and green the lowest.
The stickers will be introduced in other urban centres across Germany later this year.
Diesel powered vehicles produced more NO2 and PM than vehicles powered by petrol.
As a result, there are three stickers for diesel vehicles – red, orange and green – but only two for petrol vehicles – red and green.
Vehicles displaying any of the three stickers can currently drive into all areas, but from the start of 2010, only the cleanest – ie those displaying the green sticker – will be allowed into the new zones.
Drivers of vehicles without a valid sticker displayed in the windscreen risk a fine.
Foreign vehicles must also display the stickers, which are available from Germany’s vehicle registration offices, exhaust emission testing centres and authorised garages in the cities. They cost £3.60 (5 euros).