I think the manual has something to say about "Unset" styles. Let's see if I can find it....
Hhhm, no luck. It just says 'this is what's needed to color tiles clones' or something like that.
There are quite a few differences, at a quick glance, between an object with a regular solid color fill and unset fill. Try drawing 1 object, give it a solid color fill, and then duplicate and move to the side. Then make the duplicate Unset.
Now you can switch back and forth, while keeping XML Editor open, and it's easy to see the difference.