Some have roof billboards, some have bumper-guards, one has a digital billboard, one has body-colored bumpers.
They are all modeled after New York City's newest Ford Crown Victoria (model years 2000-2003) taxicabs. Here in Manhattan, the newest taxis are entirely yellow, while other older ones have grey bumpers and black stripes along the doors. They all have a green "badge" on their hoods. You'll also see a spattering of cabs with advertisments on their roofs, the newest of which are digital signs that change with updates from satellites!
This taxi has one of the new "electronic" billboards on the roof. In real life, these billboards light up and show everything from animated advertisements to up-to-the minute sports scores from satellite uplinks.
Here is a newer taxi cab (2002 or 2003 model year), with a roof billboard, and body-color bumpers. These newer taxi cabs no longer have grey bumpers or bumper guards, and they have black plastic grilles instead of chrome ones.
You rarely see NYC taxis with silver wheels, mostly black. You should use those chrome round studs with a thin tire around it for your wheels. They won't roll but will look correct. (They're on display, right?)