How to execute javascript within drupal for a specific field on a specific page?
Your jQuery:

//Making a focus over the element
(function ($) {
    $('#fullname').blur(function() {
        $(this).removeClass("color");
      })
      .focus(function() {
        $(this).addClass("color");
      });
})(jQuery);


Your HTML:

&lt;input type="text" id="fullname"&gt;&lt;br&gt;


Your CSS:

.color {
  background-color : yellow;
}


EDIT:
To attach the js code to your page you can use #attached in a renderable array (this is the recommended way). I think that you should read: How to add jQuery UI libraries to a page.