Kevin ,
Aluminium is a very good conductor , thus its use in electrical systems . Its tendency not to rot easily in air ,especially when alloyed , is why it is used on aircraft , though its light weight is the primary factor for airborne use .
Its propensity to dissolve in salt is legendary . Did you know that if a helicopter lands on a beach and subsequently gets sea water splashed over it , then a total write off is the only course to take ? This is what happens to non prpared helicopters , obviously the Sea king and the like are specially prepared . If an aircraft lands on a beach , there is usually more than just corrosion .
An interesting point about alu is that once the white corrosion appears , it seems to protect it . I have picked up many bits of WW2 aircraft off the beaches round the UK which , when the white stuff is removed , can reveal part numbers . That’s how shallow the corrosion is .
So , lots of folk are interested in a cheap and cheerful way of colouring Alu , many have tried , and many failed .
Etch primers are the cheapest way of protection , but they are not , to my knowledge , available in black/brown/rust . I have good connections with a paint factory local to me here , they supply paints and finishes to the Military and to the aircraft industry . they have been unable to help , except to recommend anodising . This is not a simple process , it carries some danger from the chemicals . It would not be cost effective for DIY .
You may find the odd back street shop prepared to help , but beware that anodising often involves heat . This would do your wheel insulators no good at all , should you wish to blacken them . The problem of anodising track lengths , assuming they are not embedded in plastic is the handling , oddly enough . Making it a bit labour intensive . It also gives an overall blacking , including the rail tops .
I use paint , from an airbrush .
You can get any colour you want , though I usually use rust colour. Suits me and a lot of other modellers I know .