This should prevent Outlook changing the color of visited links.

<head>
  ...
  <style type="text/css">
  span.MsoHyperlink {
      mso-style-priority:99;
      color:inherit;
  }
  span.MsoHyperlinkFollowed {
      mso-style-priority:99;
      color:inherit;
  }
  </style>
</head>