Definitely paint the entire grey area. I just restored a Taxi, pf wasn't in as poor a shape as yours so I got away with only painting the white - I used mylar on the entire pf because I don't have spray/paint application access.
Anyway, someone already restored a Taxi and used what Curbfeeler is talking about - they cut black plastic masks for all of the artwork (taxis, on the road, road lane markers, etc.) and they are an *exact* fit. Even the tiniest leaf from the tree at the entrance to the right ramp is covered effectively. Then you just spray down a grey color, peel off the masks and wha-la! Done. There's a thread on here for THAT Taxi resto also.....
Best of luck,
Nate