So it looks like this is actually happening this Autumn..
Green 'flash' number plates to encourage the switching to electric cars and avoid congestion charges
Not fully green, thankfully, but a green “flash” where the EU bar would normally go.
Looks alright to me?
EDIT: Only obvious downsides I can see are that people could put these plates on any car, and access local/national incentives (if they aren’t backed up by ANPR with a database of zero emission vehicles), and the potential increased risk of vandalism by the crowd that think electric vehicles are actually more pollutant. God help us if someone starts saying the cars have 5G in them...