Lagom does not currently support a literal colon in the URL path pattern. There is some discussion in this issue:
I suggested a workaround in the comments (encoding the colon as %3A on both the client and in the service descriptor). There are some caveats, though: if the client doesn’t encode it, and sends a literal : instead, Lagom doesn’t currently normalize this and it won’t work. I think it would be a good idea to add the normalization, and a pull request to do that would be greatly appreciated!
I didn’t test it, but if you’re using Lagom on both the client and server side, I’d expect it to work correctly.