jQueryでの条件分岐（select文）について

Question

##質問要件
生年月日を別々のselectタグで作成し、
３つのselectボックスが選択されたら、submitボタンの色を変えたい。

##HTML

    <!--ここに入力フォーム-->
    <form class="birthday_form">

      <div class="cp_ipselect select_year">
        <select class="cp_sl06" id="year" required>
          <option value="" hidden disabled selected></option>
          <option value="1">----</option>
        </select>
        <span class="cp_sl06_highlight"></span>
        <span class="cp_sl06_selectbar"></span>
        <label class="cp_sl06_selectlabel">年</label>
      </div>

      <div class="cp_ipselect select_month">
        <select class="cp_sl06" id="month" required>
          <option value="" hidden disabled selected></option>
          <option value="1">--</option>
        </select>
        <span class="cp_sl06_highlight"></span>
        <span class="cp_sl06_selectbar"></span>
        <label class="cp_sl06_selectlabel">月</label>
      </div>

      <div class="cp_ipselect select_day">
        <select class="cp_sl06" id="day" required>
          <option value="" hidden disabled selected></option>
          <option value="1">--</option>
        </select>
        <span class="cp_sl06_highlight"></span>
        <span class="cp_sl06_selectbar"></span>
        <label class="cp_sl06_selectlabel">日</label>
      </div>

      <!--ここからjs-->
        <script language="JavaScript">
          //現在の年数オブジェクトを4桁で生成
          var time = new Date();
          var year = time.getFullYear();
          //1900年まで表示
          for (var i = year; i >= 1900; i--) {
            createOptionElements(i,'year');
          }
          //1～12の数字を生成
          for (var i = 1; i <= 12; i++) {
            createOptionElements(i,'month');
          }
          //1～31の数字を生成
          for (var i = 1; i <= 31; i++) {
            createOptionElements(i,'day');
          }

          function createOptionElements(num,parentId){
            var doc = document.createElement('option');
            doc.value = doc.innerHTML = num;
            document.getElementById(parentId).appendChild(doc);
          }
        </script>
        <!--jsここまで-->
      <div class="birthday_submit">
        <input type="submit" id="submit" value="次へ">
      </div>
    </form>

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script>
    $(function() {
      if ($('#year option:selected').text() == "" ||
          $('#month option:selected').text() == "" ||
          $('#day option:selected').text() == "" ) {
              $('#submit').prop('disabled', true);
      } else {
        $('#submit').prop('disabled', false);
      }
      /*
      $('#year,#month,#day').on('keydown keyup keypress change', function() {
        if ( $(this).val().length > 0 ) {
          $('#submit').prop('disabled', false);
        } else {
          $('#submit').prop('disabled', true);
        }
      });
      */
    });
</script>

参考サイトの一部をコメントアウトして流用
##参考サイト
httpsips/jquery-switch-disable-input-button.html://www.nxworld.net/t
https://teratail.com/questions/84585

##求解答
こちらと全く同じ質問でしたが、解決方法がいまいちわかりません。
https://teratail.com/questions/84585

Answer 1

<script>
$(function() {
  
  $('#year,#month,#day').on('change',function(){
  var flg=($('#year option:eq(0)').is(':selected') ||
           $('#month option:eq(0)').is(':selected') ||
           $('#day option:eq(0)').is(':selected') );
    $('#submit').prop('disabled', flg).css("background-Color",(flg?"yellow":"lime"));
  }).eq(0).trigger('change');
  var time = new Date();
  var year = time.getFullYear();
  for (var i = year; i >= 1900; i--) {
    createOptionElements(i,'year');
  }
  for (var i = 1; i <= 12; i++) {
    createOptionElements(i,'month');
  }
  for (var i = 1; i <= 31; i++) {
    createOptionElements(i,'day');
  }
  function createOptionElements(num,parentId){
    $('#'+parentId).append($('<option>').text(num).val(num));
  }
});

</script>
<form class="birthday_form">

<div class="cp_ipselect select_year">
<select class="cp_sl06" id="year" required>
<option value="">----</option>
</select>
<span class="cp_sl06_highlight"></span>
<span class="cp_sl06_selectbar"></span>
<label class="cp_sl06_selectlabel">年</label>
</div>

<div class="cp_ipselect select_month">
<select class="cp_sl06" id="month" required>
<option value="">--</option>
</select>
<span class="cp_sl06_highlight"></span>
<span class="cp_sl06_selectbar"></span>
<label class="cp_sl06_selectlabel">月</label>
</div>

<div class="cp_ipselect select_day">
<select class="cp_sl06" id="day" required>
<option value="">--</option>
</select>
<span class="cp_sl06_highlight"></span>
<span class="cp_sl06_selectbar"></span>
<label class="cp_sl06_selectlabel">日</label>
</div>
<div class="birthday_submit">
<input type="submit" id="submit" value="次へ">
</div>
</form>

Answer 2

例えばこうでしょうか？

 <style>
   input[type="submit"]:disabled {
     background-color: #000;
   }
 </style>