Color work days headers in Google Sheets

function colorWorkdays() {
  // get the sheet we want to work with. 
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  // determine how long the sheet is
  var lastColumn = sheet.getLastColumn() + 1;
  // the sheet indexes are 1 based :-O
  var workDaysHeaderRowIndex = 1;
  
  var mondayColor = "#2fb726";
  var tuesdayColor = "#ccffcc";
  var wednesdayColor = "#b3ffb3";
  var thursdayColor = "#99ff99";
  var fridayColor = "#80ff80";
  
  // start from 3, account for the 2 columns for name and status
  for (columnIndex = 3; columnIndex < lastColumn; ++columnIndex) {
    var cell = sheet.getRange(workDaysHeaderRowIndex, columnIndex);
    var value = cell.getValue();
    var color = "ffffff";
    
    if (value instanceof Date) {
      var day = value.getDay();   // the day index in week [1-7]

      switch(day % 5) {
        case 0:
          color = mondayColor;
          break;
        case 1:
          color = tuesdayColor;
          break;
        case 2:
          color = wednesdayColor;
          break;
        case 3:
          color = thursdayColor;
          break;
        case 4:
          color = fridayColor;
          break;
      }
      cell.setBackground(color);
    }
  }
}