[Deff]

<input type="submit" omsubmit="document.getElementById('phonered').style.background='red';" name="send-request" class="btn" value=" " />

Или

<script type='text/javascript'>
var phoneobj;
function setStyle(){
  phoneobj = document.getElementById('phonered')
  phoneobj.style.background='red';
}
</script>


<input type="submit" omsubmit="setStyle()" name="send-request" class="btn" value=" " />