I agree, it's not worth re-painting it. As far as I remember, the red colour was the self-sealer outer coating, which was a rubber-based coating, the name of which escapes me at the moment (the material, in sheet form, is still used as a back stop on small bore shooting ranges). If pierced, and then in contact with fuel, it swelled up as a reaction, thus blocking, or at least partly blocking, the puncture.
From what I remember of the BBMF aircraft, the 'silver' colour is a 'modern' protective coating, but not for sealing purposes. Note that, in line with current RAF practices, the BBMF Spitfires have the internal areas of the rear fuselage painted white, partly to disclose any leaks (internal or external) or other contamination, and partly for ease of visibility when servicing.