It's a very vague question, understandably! First thing is what's permissible varies from country to country; and I mean both legal and what you might consider culturally acceptable. Certain ideas and things may be legal, but not a good idea - and vice versa.
k5083 wrote:
My sense is that many Spitfire operators in the UK went with military colors right from the beginning.
Good post, August. However, IIRC in the 1950s most, if not all UK ex-military types were in civil schemes. Swordfish and Fulmar were in Fairey colours, the Hawker Hurricane and Tomtit of Hawkers were in civvies (the Hawker Hart went military) and there were several Spitfires privately owned in civil schemes with registrations painted on, from Mk.I, II and the Rolls Royce XIV G-AGLT.
IIRC, today in the UK you need an exemption to paint an aircraft in a military scheme without showing the civil registration. You can, I think paint it as you wish without military markings, but showing the registration. In Australia it's something similar, but they carry the reggo in small under the tail.
Leaving aside the usual "it's mine to do as I wish" statements, ex-military aircraft would normally carry genuine and sometimes still current military identification markings. For that reason, rightly and understandably, most militaries and governments are wary of letting anyone carry those on the private machine. It is like using government headed paper.

A good example is the ex-Canadian T-33. I'm reasonably up to date on warbirds, but if Paul turned up in it locally, I wouldn't know it wasn't still a Canadian military operated aircraft.
We all know Paul's a real gent, but the chicks need to be aware he's not a nice Canuckian air force pilot, but an unleashed private owner.
I've always like a well thought-out civil scheme on a warbird, and the y can look great. G-FIRE and the Flack stable in the UK were one of the latest, greatest examples of that.
HTH