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

function checkInput(form) {
  var bgBad = "#FFE1E1";
  var bgGood = "#FFFFFF";
  var valid = true;
if (form.USER_ID.value == "") {
    valid = false;
    setColor(form.USER_ID, bgBad);
  } else {
    setColor(form.USER_ID, bgGood);
  }
if (form.PASSWORD.value == "") {
    valid = false;
    setColor(form.PASSWORD, bgBad);
  } else {
    setColor(form.PASSWORD, bgGood);
  }
  if (!valid) alert("Sie haben noch nicht alles richtig eingegeben!");
  return valid;
}