Untitled

<input type="text" name="promo" autocomplete="off" class="promobox" id="promobox" placeholder="PROMO CODE" onkeyup="this.setAttribute('value', this.value);" />

.promobox {

  color: #800;

}

.promobox[value="VALIDCODE"] {

  color: #080;

  font-weight: bolder;

}