c5 product customisation
If you're using IE (8 or above, I think), you can use its built-in developer tools to do the same thing. Just hit F-12 to turn it on.
At any rate, that would be one way to do it. That is, create another version of header.jpg that is the color you want. Just about any photo/image editing program should be able to handle that.
Or, if you just want a solid color, you can just use CSS to create an area the same size and position where the background image is now, then make its background color whatever you want, using CSS properties.