Well as its an unregistered colour ie doesn't belong to a specific nation then I'd guess you can have what ever you want its simply decoration isn't it?, now having Spanish flag on a UK car for eg I'd say would be breaking some rule,or other..
Definitely not legal.
The only addition to a standard white or yellow number plate that is legal is the blue Euro badge. Anything else at all (St George cross, Saltire etc) is illegal.
You can buy them of course, but they will be sold as 'show plates' to get around the law. You are also highly unlikely to ever get done for it, so it all depends how scrupulous you are about keeping the right side of the law!
Thanks guys. I didn't choose any flag . Just sent for green border. Simples. Thanks.