Welcome to the Bunnings Workshop community @Dazzag. It's terrific to have you join us, and many thanks for your question about installing decorative screens.
I can't think of any particular rails which would allow you to fix you screens onto a Colorbond fence without screwing into it.
If you are planning on having these rails and support structure completely independent of the fence, you'll need to fix it into the ground. A very simple method would be to concrete Protector Aluminium 50 x 50 x 2100mm Black Inground Fence Posts into the ground where the screens meet and at either end. The footers won't need to be particularly large as the post aren't support much weight. A 200 x 200 x 300mm footer should be sufficient.
The posts are 50mm wide, so you'll get 25mm of each screen onto the post and can screw through their face into the post with Tek screws.