[{
  "GroupName": "default",
  "Priority": 1,
  "Title": {
    "Text": "[Player]",
    "Color": "#55aaff",
    "Size": 15,
    "Hidden": true,
    "HiddenIfNotPrimary": false
  },
  "Username": {
    "Color": "#55aaff",
    "Size": 15
  },
  "Message": {
    "Color": "white",
    "Size": 15
  },
  "Format": {
    "Chat": "{Title} {Username}: {Message}",
    "Console": "{Title} {Username}: {Message}"
  }
}, {
  "GroupName": "Admin",
  "Priority": 0,
  "Title": {
    "Text": "[Admin]",
    "Color": "#ff0a0a",
    "Size": 15,
    "Hidden": false,
    "HiddenIfNotPrimary": false
  },
  "Username": {
    "Color": "#65ff57",
    "Size": 15
  },
  "Message": {
    "Color": "white",
    "Size": 15
  },
  "Format": {
    "Chat": "{Title} {Username}: {Message}",
    "Console": "{Title} {Username}: {Message}"
  }
}]​

I recommend setting "Hidden" to true for the title instead.

Make sure your reload the plugin if you've changed the file manually. Also make sure it's valid with a tool like jsonlint.com/

RyanFromRust

How are you sending the file back? Are you reloading the mod once changes are sent back?

And make sure you unloadany plugin before editing its data file ,  unload, edit, save, reload