Hello,
You just need to specify the colour in your skin.css file, if you have a look at the skinning toolkit at .
skinobject you can see how the styling of the breadcrumbs works. If you want to have unique styling for the breadcrumbs, you can specify the class that they should use instead of the skinobject class.
If you have created a skin using a html file this will be defined in the skin.xml file, if you have created a skin using a .ascx file the class is defined directly in that file.
You could setup a background colour using the body class, but it depends on the structure of your skin. - If you have a link to the skin it's much easier for us to view live and offer help than look through pasted code.
Hope that gives you a start, thanks,