In response to this post, I created a negative overlay filter and uploaded it to Open Clip Art. When applied to an object, the object's color renders as the negative of whatever is underneath it. Here's an example, with the filter applied to the text object:

This Filter is cool too.
Today Johnny Automatic added a Filter Tab to the menu. So if you create a new Filter use the tag: Inkscape_filter to list it there.

Thanks for sharing.