// regex for hex color codes
HEX_COLOR_REGEX = /(^#[0-9A-Fa-f]{3}$)|(#[0-9A-Fa-f]{6}$)/;

// column to watch for changes (i.e. column where hex color codes are to be entered)
HEX_CODE_COLUMN = 1; // i.e. column A

// column to change when above column is edited
HEX_COLOR_COLUMN = 2; // i.e. column B

// utility function to test whether a given string qualifies as a hex color code
function hexTest(testCase) {
  return HEX_COLOR_REGEX.test(testCase);
}

function onEdit(e) {
  var range = e.range;
  var row = range.getRow();
  var column = range.getColumn();
  if (column === HEX_CODE_COLUMN) {
    var values = range.getValues();
    values.forEach( function checkCode(rowValue, index) {
      var code = rowValue[0];
      if (hexTest(code)) {
        var cell = SpreadsheetApp.getActiveSheet().getRange(row + index, HEX_COLOR_COLUMN);
        cell.setBackground(code);
        SpreadsheetApp.flush();
      }
    });
  }
}