What is the problem with posting to /user/signup that posting to /user:signup solves?
So then one gets to bike-shed if "signup" it is in the request path, query parameters, or the body. Or that since the user resource doesn't exist yet perhaps one can't call a method on it, so it really should be /users:signup (on the users collection, like /users:add).
Provided one isn't opposed to adopting what was bike-shedded elsewhere, there is a fairly well specified way of doing something RESTful, here is a link to its custom methods page: https://google.aip.dev/136. Its approach would be to add information about signup in a request to the post to /users: https://google.aip.dev/133. More or less it describes a way to be RESTful with HTTP/1.1+JSON or gRPC.
But that's not a difference between /user/signup and /user:signup .
I assumed most readers of my comment would get that the idea that /users/signup is ambiguous whether or not that is supposed to be another resource, while /users:signup is less so.
what's the confusion? you're creating a new user entity in the users collection.