Hello,
I am on a holiday in France. And some license plates is yellow and some are white. Whats the difference?

They changed a few times:
- white (or silver) on black in the 50s
- black on white (front) or yellow (back) allowed since the 60s, mandatory since the 90s
- black on white (both front & back) allowed since 2007
and they changed the numbering style in 2009 (now they are all black on white, I think)