You could do this a number of ways. Since you have 15 you could use the renovation settings to create a different 3d material for the existing/new objects. You could also just change the materials for those buildings you don't want to emphasize with white, great, glass or ice. I'm sure other people will post some good ideas.
matt johnson
archicad since 2004
imac 27, 4.2 GHz Intel i7, 16 GB RAM, radeon 575 4 GB, macOS 10.14.6
imac, 4.0ghz i7, 16gb ram, Radeon M9 390 2GB, OS X El Capitan