var popup = null;

function openPopup(id) {
  if (popup) {
    closePopup();
  }

  popup = document.getElementById(id);
  popup.style.visibility = "visible";
}

function closePopup() {
  popup.style.visibility = "hidden";
  popup = null;
}

function hoverColor(element, color) {
  element.style.backgroundColor = color;
}