Well I don't even know how to find a place to edit the window that pops open when you tap on a figure. That seems to be something you can't edit.
And if I select the figure in the regular page view, Fill is grayed out. Only Stroke and Shadow are active.
I guess I could always had a shape behind the graphic that's the color I want it to be, but I think that would begin to add up quite a bit on the file size.