If you want to know the exact law...
Check out the The Road Vehicles (Display of Registration Marks) Regulations 2001
http://www.legislation.gov.uk/uksi/2001/561/contents/made
PART 2
VEHICLES REGISTERED ON OR AFTER 1ST JANUARY 1973 AND BEFORE 1ST SEPTEMBER 2001 (OPTIONAL SPECIFICATION)
1. The plate must be made of reflex-reflecting material which, as regards its construction, colour and other qualities, complies with the requirements of—
(a)the British Standard Specification for reflex-reflecting number plates, published on 11 September 1972 under the number BS AU 145a(2), or.
(b)any other relevant standard or specification recognised for use in an EEA State and which, when in use, offers a performance equivalent to that offered by a plate complying with the British Standard specification,.
and which, in either case, is marked with the number (or such other information as is necessary to permit identification) of that standard or specification.
2. Where the registration mark is displayed on the front of the vehicle, it must have black characters on a white background.
3. Where the registration mark is displayed on the back of the vehicle, it must have black characters on a yellow background.
Hope that helps.
GP