The primary rainbow is caused from one reflection inside the water droplet. The secondary rainbow is caused by a second reflection inside the droplet, and this “re-reflected” light exits the drop at a different angle (50° instead of 42° for the red primary bow). This is why the secondary rainbow appears above the primary rainbow. The secondary rainbow will have the order of the colors reversed, too, with red on the bottom and violet on the top.

Drawing shows viewing angle for primary and secondary rainbows.