function setColor(el, bg) {
  if (el.style) el.style.backgroundColor = bg;
}

function checkInput(form) {
  var bgBad = "#FFC000";
  var bgGood = "white";
  var valid = true;
  if (form.email.value == "") {
    valid = false;
    setColor(form.email, bgBad);
  } else {
    setColor(form.email, bgGood);
  }
  if (!valid) alert("Please fill out the mandatory fields.");
  return valid;
}