[
  {
    "GroupName": "Default",
    "Priority": 1,
    "Title": {
      "Text": "[Player]",
      "Color": "#55aaff",
      "Size": 15,
      "Hidden": false,
      "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": "#F21905",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": false
    },
    "Username": {
      "Color": "#F21905",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  }
]​

Please use lower-case group names, e.g. "default" and "admin". That should also fix your issue.

However, you can't remove yourself from the default group. More information about that can be found on the FAQ on the plugin page.