You could do that with a screenscraper, but it would be a bit more advanced.
First it would scan the site for the stylesheet - not hard.
Then it would open the stylesheet.
Then it would parse the stylesheet and get the colors.
A screenscraper is what you want to build. I would recommend playing with PHP screenscrapers that use cURL or somesuch.
You can probably hire someone to build it on eLance, but this is a more involved project than a simple screenscraper and nobody here will write one for you for free.
Perhaps if you got it started by getting it to extract the CSS Stylesheet URL.
You might interest someone into building a CSS analytic tool with you.
You might want to play with this script: