Ah, starting with the easy questions first eh?
The 'O' is that aircraft's identity within the squadron, same as the WW2 RAF method of marking, DW-D for one of Al Deere's Spitfires, for instance.
I can see from the underside of the wing that the aircraft was part of the 'VP' batch, so lets say it's VP999, for instance. The large '139' is the fleet number (I think that's the correct term) and bears no relationship to the aircraft's actual identity, VP999. I hope that makes it clear.
As for colours, You could have as above, extra dark sea grey & sky. Or, extra dark sea grey down the sides (again, similar to WW2 RAF spitfires) so that only the underside is sky. At one point jets were very dark blue over white. So, with your ficticious P 40, you can have almost any Naval type scheme.
I hope at least some of this helps.
Pete