I bought one of the collars for my dog with "No Dogs" on it, basically they make them in three colours, Green with "Safe" on on for a good dog, Orange, like mine, for dogs not good with other dogs, and Red with "Caution" on it for dogs not good with other dogs or unknown people.
Like a lot of things it's a good idea if people actually know what it means, mine just look like I'm protesting if you've never seen one.
As an aside, I was always told my dog was no good with other, and the first few months seemed to back that up hence the middle of the night walks. The sunny weather means he's had a few earlier ones, and now I have to amend that to: he doesn't like MALE dogs. (You can work the rest out yourself).