Path to theme within a module / jQuery context?
Use Drupal.settings.pathToTheme.

$("li.green").click(function () {
 $('#logo-img').attr('src', Drupal.settings.pathToTheme + '/images/logo-purple.png');
});


Also, theme_preprocess_page(&amp;$vars) should be in template.php as your_theme_preprocess_page(&amp;$vars);

Good idea to not hard code the theme path, that path structure wouldn't work for all set-ups.