I have always assumed that the reason for a common colour was to do with economies of scale, linked with ease of identification. The South London properties, in an urban area, include presumably rented homes. I initially gathered from the press that the door colour was of the same pattern.

I think it has been quite easy for people not to think of what vicious thugs might make of something, because most people do not mix with said thugs. It wouldn't have occurred to me.

Anything of the sort introduced now, of course, would show deliberate choice in the knowledge of what can happen.