If you use macromedia Dreamweaver to insert the swf in your web page you can just select the swf file and change the background color at the bottom of screen in "properties panel".
If you want to manually locate and change the html code for the swf color locate and search for: "BGCOLOR" parameter, note that you have to edit it in 2 places in the code, in the "param" tags and in "embed" tag. Replace the color code with the code of the color you need.
