function myFunction() {
    var y = document.getElementById("txt1").value;
    var z = document.getElementById("txt2").value;
    var x = parseInt(y) + parseInt(z);
   // document.getElementById("demo").innerHTML = x;
  document.getElementById("txt3").value= x;
  var elem = document.querySelector('#txt3');

  if(x < 50)
// Set color to purple
elem.style.color = 'red';
 else
     // Set color to purple
elem.style.color = 'green';
  }