Ah, OK.
Now that does make sense. Though it somewhat leaves open the question of why the front ones are yellow, norfolk's answer at 15:47 notwithstanding.
( Enzyme is powered by sunlight, Wed 19 Oct 2011, 16:20, 2 replies)

That probably is about all there is to it
Black on white is the most clearly legible, but you can't have white at the back so they specify the next most clearly legible, which is presumably black on yellow.
(richardm, Wed 19 Oct 2011, 16:55, closed)

Norfolk is right
Yellow is the next best reflective colour and gives the best contrast with black letters.

This is one reason why military vehicles still use the white on black number plate style - they don't reflect and can't be used for laser targeting.
(azgiles, Wed 19 Oct 2011, 17:00, closed)

Military vehicles
If you look under the back you'll see that the diff housing is painted white. Somewhere up on the chassis is an extra "numberplate light" that can be switched on seperately, so that when they're driving in convoy with all their lights off they can switch on this little 1W light underneath to light up the white paint, so the guy behind can see where they are. Because the wheels are in the way, you can't see it from the sides.
(gordonjcp, Wed 19 Oct 2011, 17:46, closed)

Is that why that is?
I'd often wondered about that.
(richardm, Thu 20 Oct 2011, 9:29, closed)