mooColorFinder can search through the CSS files for any color used in that website, as long as a website specifies its CSS styles W3C conform.
Simply type in the website address and hit enter. After that you will get a screenshot of the website, including all colors used and its color code.
The above screenshot show the result of colors used in this page. mooColorFinder also provide an API for users to display the color cloud on their page.