Changing background color of specific cell on Google sheet

You can use setBackground property with getRange. Try the below snippet.

function myColorFunction() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var range = ss.getSheetByName("Form Responses 1").getRange(2,6,ss.getLastRow());
  var cellRange = range.getValues();

  for(i = 0; i<cellRange.length-1; i++){
     if(cellRange[i][0] == "Open")
     {
       ss.getSheetByName("Form Responses 1").getRange(i+2,6).setBackground("red");
       ss.getSheetByName("Form Responses 1").getRange(i+2,6).setFontColor('white');
     }
  }
}