Call get_option from php outside wordpress

I am trying to create a way for a template user to modify the colors without going into the css.

Storing the colors and retrieving them is easy, and get_option is working to retrieve the colors, but I want to be able to call get_option from a php file that renders the css. i.e. the file would be colors.php and linked to in the header.php file.

How can colors.php call get_option? Are there files I can include at the top of the colors.php to obtain that functionality?

Thanks,

Andrew