"Fine-dust" permit for the cities

From 1 Oct 2008, every car on the road in Germany entering a city zone needs to display a "fine-dust" emission sticker, otherwise a 40 Eur fine. Ours is 4-green, there is 3-yellow and 2-red.  From 2010 only the green & yellow stickers will be allowed in the city zones, and then from 2012 only green stickers are allowed.