In 1970 Michigan's standard plate had white letters on a yellow (technically, gold) background that was impossible to distinguish even in the daytime. The police hated it.

Again in the early 2000s it had an optional specialty Mackinac Bridge plate that featured white on an orange background. It again caused problems and was withdrawn, replaced by a similar design with black lettering and a lighter background.

They can be seen on this page: http://www.worldlicenseplates.com/usa/US_MIXX.html