You can customize WordPress admin bar. You can change its color, size and other style by adding some custom CSS. Generally this is done by adding a function to your theme’s function.php file. Second method includes editing siteroot-> wp-includes->css-> admin-bar.css file but I would not recommend you changing the core css files. As far I know there are no plugins available that allows you add custom color to wp admin bar. Here I am discussing another method which easily customizes your wp admin bar without disturbing anything on your site.