The worse for mismatched panels paint wise is apparently the Ferrari Enzo in red!
I used to have the European plates on my Focus way back before I put the custom plates on and started heavily modifying the look of the car. They were so wide though, they made the car look tall and skinny because they were so wide. That's the reason I got rid of them anyway, of course other people have other opinions.
The worst part was that Ford just self tapped the plates straight into my front bar when they installed the wide plates, so the holes had to be filled and painted as they were in plain view with custom plates on it. Fortunately I was up to the installation of the front bar, requiring a front end respray that took out all my stone chips too, so that wasn't the end of the world.
Interesting to see they are up to XJ plates now, I had XC plates (edit - obviously, with the photo right there).
Tim