Blocking cell after changing it's color

Question

I have created simple formula in google sheets which is going to be shared with many people. Sheet will be secured except few columns. I want to let those people edit cell's color in those columns. However, I don't want another person to change cell's color if it was already changed. Is there a way to block cell so it won't be allowing color changing? I found such a script:

function onEdit() {
  // This script prevents cells from being updated. When a user edits a cell on the master sheet,
  // it is checked against the same cell on a helper sheet. If the value on the helper sheet is
  // empty, the new value is stored on both sheets.
  // If the value on the helper sheet is not empty, it is copied to the cell on the master sheet,
  // effectively undoing the change.
  // The exception is that the first few rows and the first few columns can be left free to edit by
  // changing the firstDataRow and firstDataColumn variables below to greater than 1.
  // To create the helper sheet, go to the master sheet and click the arrow in the sheet's tab at
  // the tab bar at the bottom of the browser window and choose Duplicate, then rename the new sheet
  // to Helper.
  // To change a value that was entered previously, empty the corresponding cell on the helper sheet,
  // then edit the cell on the master sheet.
  // You can hide the helper sheet by clicking the arrow in the sheet's tab at the tab bar at the
  // bottom of the browser window and choosing Hide Sheet from the pop-up menu, and when necessary,
  // unhide it by choosing View > Hidden sheets > Helper.
  // See https://productforums.google.com/d/topic/docs/gnrD6_XtZT0/discussion

  // modify these variables per your requirements

  var masterSheetName = "master" // sheet where the cells are protected from updates

  var helperSheetName = "Helper" // sheet where the values are copied for later checking

  var firstDataRow = 1; // only take into account edits on or below this row

  var firstDataColumn = 1; // only take into account edits on or to the right of this column

  var ss = SpreadsheetApp.getActiveSpreadsheet();

  var masterSheet = ss.getActiveSheet();

  if (masterSheet.getName() != masterSheetName) return;

  var masterCell = masterSheet.getActiveCell();

  if (masterCell.getRow() < firstDataRow || masterCell.getColumn() < firstDataColumn) return;

  var helperSheet = ss.getSheetByName(helperSheetName);

  var helperCell = helperSheet.getRange(masterCell.getBackgroundColor());

  var newValue = masterCell.getBackgroundColor();

  var oldValue = helperCell.getBackgroundColor();

  var green = "#00FF00"

  var red = "#FF0000"

  if (oldValue == green) {
    masterCell.setBackgroundColor(green);

  } else {
    helperCell.setBackgroundColor(red);
  }
}

But it doesn't work. Any idea?