The background color does not work when generating a .gif. It doesn't matter what color you specify, the background is always generated as transparent. I think this is an error with the tool somewhere.
this tool is actually designed for generating transparent GIF. the reason we provide a background color option is to pre-blend gif with the background color where we are going to use this loader, so that the edge of GIF will looks better when we actually use it.
yet I can still add an option to preserve background color... maybe later when I have time :p