<form action="" method="post" role="form" accept-charset="utf-8" class="testform" >
      <legend>test</legend>
      <div class="form-group">
        <label for="username">username :</label>
        <input type="text" id="username" class="form-control m-20">
      </div>
      <div class="form-group">
        <label for="password">password :</label>
        <input type="password" id="password" class="form-control m-20">
      </div>
      <div class="form-group">
        <label for="repassword">re-type password :</label>
        <input type="password" id="repassword" class="form-control m-20">
      </div>
      <div class="form-group">
        <label for="email">Email :</label>
        <input type="Email" id="email" class="form-control m-20">
      </div>
      <button id="send" class="btn btn-primary">
        submit
      </button>
    </form>

// button submit got clicked
$('#send').on("click", function(){
    var input= $('.testform input');
    var empty= $("<span class="error">empty</span>");
    // check input if have value if empty
    input.each(function() {
        var self=this;
        var value=$(this).val();
        // if empty change color of label
        if(value ==""){
            // $(self).parent(".form-group").find("label").prepend(empty);
            $(self).parent(".form-group").find("label").css({"background": "green"});
        }else{
            $(self).parent(".form-group").find("label").css({"background": "white"});
        }
    });
    return false;
});