Hey there.
Deciding when and what to paint is a dilemma - it's original only once. After that it depends on what you are trying to achieve. If you have original Gravelys and want matching components, then you may want to paint them with a custom colour to match the faded tractor.
My opinion, choose the colour that reflects you. Maybe grey to match your Ford?
You're already doing a great thing by taking the long road and making it "right". Not letting it go to scrap metal or just hosing down the rust with tremclad.
I'm trying to decide if I should paint my 1960 Bolens Ride-a-matic. So far, not painted reflects my lack of decisiveness.