The old plates were white on black. The new ones (new being like thirty years ago) are white over black or white over yellow depending on the positioning on the vehicle. There's no rhyme or reason to it except that those colors are quite visible and work well with the reflective materials used on the plates. You'll note that most European countries use black on white, Belgium being the most notable exception. Holland (Netherlands) uses black on yellow. It's just easiest to see.
Dark over light is typically more see-able than light over dark, although the NHTSA did seven zillion tests years ago to determine the best color for highway signs and decided that white over green was indeed best - they've been using it ever since. Most US license plates are dark numerals over a light background as well, with the exceptions being Delaware, Michigan, and Vermont.