Change the color of a button if at least one option is chosen from a tag

I have a select with several options and a gray button that pretends to be disabled. I want that if at least one option is chosen, the button will change color.

And this is my function, but I can’t make it change the color of the button

var options = document.getElementById('months').selectedOptions;
var value = Array.from(options).map(({
  value
}) => value);
if (value.length > 0) {
  let element = document.getElementById("btn_next");
  element.classList.remove("color");
  element.classList.add("color2");
}

.color {
  background: gray;
}

.color2 {
  background: orange;
}

<form>
  <select id="months" name="months" multiple>
    <option value="January">January</option>
    <option value="Fabruary">Fabruary</option>
    <option value="March">March</option>
    <option value="April">April</option>
    <option value="May">May</option>
    <option value="June">June</option>
    <option value="July">July</option>
    <option value="August">August</option>
    <option value="September">September</option>
    <option value="October">October</option>
    <option value="November">November</option>
    <option value="December">December</option>
  </select>
  <input id="btn_next" type="button" class="color" value="NEXT">
</form>