Highlight a character name (5argon : Hello world!) or any lines that begin with # sign. Each character get different colors via the function at the bottom.

GoogleDocsHighlighter.gs

/**
 * @OnlyCurrentDoc
 *
 * The above comment directs Apps Script to limit the scope of file
 * access for this add-on. It specifies that this add-on will only
 * attempt to read or modify the files in which the add-on is used,
 * and not all of the user's files. The authorization request message
 * presented to users will reflect this limited scope.
 */
 
/**
 * Creates a menu entry in the Google Docs UI when the document is opened.
 * This method is only used by the regular add-on, and is never called by
 * the mobile add-on version.
 *
 * @param {object} e The event parameter for a simple onOpen trigger. To
 *     determine which authorization mode (ScriptApp.AuthMode) the trigger is
 *     running in, inspect e.authMode.
 */
function onOpen(e) {
  DocumentApp.getUi().createAddonMenu()
      .addItem('Start', 'formatInkVN')
      .addToUi();
}

/**
 * Runs when the add-on is installed.
 * This method is only used by the regular add-on, and is never called by
 * the mobile add-on version.
 *
 * @param {object} e The event parameter for a simple onInstall trigger. To
 *     determine which authorization mode (ScriptApp.AuthMode) the trigger is
 *     running in, inspect e.authMode. (In practice, onInstall triggers always
 *     run in AuthMode.FULL, but onOpen triggers may be AuthMode.LIMITED or
 *     AuthMode.NONE.)
 */
function onInstall(e) {
  onOpen(e);
}

function formatInkVN() {
  var body = DocumentApp.getActiveDocument().getBody();
  
  //all text!
  var allText = body.editAsText();
  
  regexAction(allText, "^.*[ ]:[ ]", formatCharacterNames);
  regexAction(allText, "^#.*", formatHashtags);
}

function regexAction(allText, regexString, actionOnText)
{
  var rangeElement = allText.findText(regexString);

  while( rangeElement != null)
  {
    var matchedText = rangeElement.getElement().asText();
    var begin = rangeElement.getStartOffset();
    var end = rangeElement.getEndOffsetInclusive();

    rangeElement = allText.findText(regexString, rangeElement); //continue from previous point
    actionOnText(matchedText,begin,end);
  }
}

function formatCharacterNames(matchedText,begin,end){
    matchedText.setBold(false); //matchedText is the entire line, I want to reset the line first
    matchedText.setForegroundColor("#000000");
    
    matchedText.setBold(begin, end, true); //Then I bold only the part that RegEx matches.
    var characterName = parseCharacterName(matchedText, begin, end);
    matchedText.setForegroundColor(begin,end, characterNameToColor(characterName)); 
}

function formatHashtags(matchedText,begin,end){
    matchedText.setBold(begin, end, false);
    matchedText.setForegroundColor(begin,end,"#BBBBBB"); 
}

function parseCharacterName(text,begin,end)
{
  var name = text.getText().substring(begin,end-2); //remove 3 characters from the end " : "
  //Assuming in the parentheses is the character's real name e.g. SunnyDay (Shun) -> Shun
  var matchArray = name.match(/\((.+)\)/);
  Logger.log(name);
  Logger.log(matchArray);
  if(matchArray != null)
  {
    return matchArray[1]; //index 0 is the original string
  }
  else
  {
    return name;
  }
  
}

function characterNameToColor(characterName)
{
  var mapper = {
    "Reef" : "#513d04" ,
    "Sui" : "#0e7b93" ,
    "Shun" : "#7d9e2a" ,
    "Ef" : "#202463",
    "Ave:ra" : "#b5972b",
    "Hydrangea" : "#665470",
    "Lua" : "#6b1a17",
    "Eru" : "#e2a548"
  }
  var color = mapper[characterName];
  if(color == null)
  {
    return "#000000";
  }
  else
  {
    return color;
  }

}