On the large train layout that Glazz had on display in Seattle most of the city sections are grey. The exception is an area with houses and the garden center that is green. You might paint it grey, then have some green construction paper or cloth sections that can be laid down in places.
I think grey is a good starting point for City Life, Mark, and as you say green can be added afterwards, maybe even as late as Friday using emulsion or acrylics.
I had a thought today and acted upon it. The Scout Hut has a TV and dvd player. I have seen Playmobil commercials on the TV so I dropped an e-mail to Jamie at Playmobil asking if he had any TV ads on dvd. He is sending a disc in the post.Mark