object.style.borderTopColor = "red";

<style type = "text/css">
   p.example1 {
      border:1px solid;
      border-bottom-color:#009900; /* Green */
      border-top-color:#FF0000;    /* Red */
      border-left-color:#330000;   /* Black */
      border-right-color:#0000CC;  /* Blue */
   }
</style>

<p class = "example1">
   This example is showing top border in red color.
</p>