<form method="POST" action="User.do">
    <div id="buddy-form-group">

          <input type="text"
             class="form-control form-input-field form-interests-input-field"
             name="interests"
             onchange="checkFilled()">

     </div>

     <br/><br/><button class="btn btn-success" type="submit">Send</button>
</form>

function checkFilled()  {
    var interests = document.getElementsByClassName("form-interests-input-field");
    for (var i = 0; i<interests.length; i++)  {
        if (interests[i].value = "")  {
            interests[i].style.backgroundColor = "red";
        }
        else  {
            interests[i].style.backgroundColor = "green";
        }

    }
}

<script>
    function checkFilled()  {
        var interests = document.getElementsByClassName("form-interests-input-field");
        for (var i = 0; i<interests.length; i++)  {
            if (interests[i].value == '')  {
                interests[i].style.backgroundColor = 'red';
            } else {
                interests[i].style.backgroundColor = 'green';
            }
        }
    }
</script>

<form>
    <div id="buddy-form-group">

          <input type="text"
             class="form-control form-input-field form-interests-input-field"
             name="interests"
             onkeyup="checkFilled()">

     </div>

     <br/><br/>
     <button class="btn btn-success" onclick="checkFilled()" type="button">Send</button>
</form>