Agree with all on the steel, Daniel. Apart from the possible rust problem, there's also the weight issue. If you can get hold of enough fibreglass sheet/panels, that's the way to go, providing you can attch it O.K. Apart from being lighter, it will resist corrosion and wear. If you cant get the fibreglass, try the printing plate method. As for the interior green/Zinc Chromate, I could try posting the colour, but I doubt it would show up well on the screen. If you want the interior, 'cockpit green' type colour, find a mid green paint and add yellow, until it looks like an 'apple' green, with a hint of yellow. I can only give you Humbrol numbers as a guide, which you can use to get some household emulsion or enamel paint, in large enough tins, much, much cheaper than buyimg modelling paints.
The Humbrol MATT colours are :- Dark Green No 30, Trainer Yellow No. 24.
By mixing the OPPOSITE way, i.e. some green into the yellow, you'll get an approximation of the sickly yellow-green Zinc Chromate, used in areas of the internals not 'occupied' by crew/pilot. In other words, inside the rear fuselage, inside cowlings and wheel bays etc. You don't need the 'real thing', which would of course serve its purpose by stopping corrosion, but it costs nearly as much per tin as the undercart on an A4 Skyhawk!! (slight exegeration).
Terry.