I am trying to figure out how the background of this image would be done in inkscape. I believe that you would either use the clip or mask function in inscape, but for some reason neither of those seem to work when I trying them. Is there another way?

I imagine the best method would be to create the slanted rounded rectangle first, and then do a difference boolean to get the silhouette of the girl out, and then perhaps break the graphic apart and change the color of the top half.

Why aren't clips working for you? What is happening?

Note when clipping multiple objects, you need to group them first. Also, note that the girl object will need to be a solid object with the girl cutout as a hole.

Note when clipping multiple objects, you need to group them first

Did not know this and as a result I would get some unpredicted results which did not resemble either a clip or mask. That solved the problem.