Why are the registration plates on the back of british cars yellow while the front ones are white?

Older UK plates were non reflective silver on a black background

Plates at the moment are reflective, black on a white background at the front and black on a yellow background at the rear.

I think this was introduced so that they were easier to read from a distance.

I believe classic cars are exempt from this, and can still use the old style

The differential may have been used so that the police can tell if you are coming or going if you are spotted in the distance!