The seats were simply known as "plastic," and were a mix of resin and paper. They were intended for use in 1938, but so much trouble, due to cracking, was experienced, that they didn't come into use until May, 1940, and then as an extra, not a replacement. They were supposed to be used exclusively by Castle Bromwich, but became universal. They were never bakelite (this stems from some office-bound wallah using the word in the manual,) nor is it the latest favourite, Tufnol (same city, Glasgow, but a different company,) though the manufacturers do make a material which sounds similar. For several years, Humbrol made a match for Spitfire interior green, 90 beige-green, but the reason for it got lost in the mists of time, and Hornby formed the impression that 90 was supposed to be Sky, so they've now changed the formula to match a "wrong" colour. One other detail, the green was not a primer; it was actually sprayed over a grey undercoat.
Edgar