I was chatting to a Wiltshire Traffic Officer about putting a black and silver ageless plate on a trad Mog and although he said it isn't allowed I got a strong impression that, at least in Wiltshire, as long as the ANPR camera gets a good clear read they are not overly fussy.
BUT as has been said, you buy a licence to use a registration number and DVLA can withdraw it should you abuse the number. Inappropriate use of a Black and Silver plate could be considered to be such an abuse.
