If it is buying matching paint you are concerned about, Ford dealers have small containers of paint in the parts department. They should be able to match it better than anyone. I'd ask them about the primer as well.
Once painted, the spot will stand out due to age differential and lack of oxidation the surrounding are has incurred. But if you don't suffer from OCD and can live with a little discrepancy out of sight on the roof as long as the rust stopped it will be fine.
I'd bet that you can find something on youtube addressing a home remedy.
