Are you trying to change the background color of your site or the text colors?
While modifying your skin, it helps to disable server and browser caching so you can see your changes immediately. To disable the caching, add the following to your user.config file between the <appSettings></appSettings> tags. After adding the lines to the user.config, open the web.config, add a space to the bottom and save it.
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />