Could you give an example of an ExecStart line that uses a colon? I haven't found any documentation for that while using Google and I don't have examples of it in my systemd unit files.

Yup, it's undocumented for some reason. I don't remember where I saw it used, but as an example

  [Service]
  ExecStart=/bin/echo $PATH

  [Service]
  ExecStart=:/bin/echo $PATH