I read the official tutorials but haven't found this- I have a curved line in the middle of the page stretching from left to right edge.
I would like the page above the line to be in color A (let's say red) and in color B below the line (let's say green). Two won't do because I want color to follow the shape of line...

Thanks

Create a rectangle the size of the page (use snapping to page to make it precise), duplicate the line (make sure that the rectangle is behind the line), then use Path > Division (Ctrl + /). This will create two objects that you can style differently.

Wow, great, tnx