OK, this is actually a rather easy question to answer: If you've ever seen tarnished brass (old brass musical instruments will show tarnish were the lacquer clear coat has been worn off, for example, or perhaps an older brass table lamp), it "tarnishes" due to exposure to air. Tarnish is different than corrosion, in that brass, being an alloy of copper and zinc, can corrode upon contact with chemicals, even water, which is a bluish-green color (Turquoise, the semi-precious gem stone, is actually the result of corroded copper in nature). Early, so-called "brass era" automobiles used polish brass trim and radiators simply because the technology of really good electroplating hadn't yet been developed, although nickel was used on occasion. However, nickel doesn't stay bright on its own very well at all--goes dull on exposure to air, and being a fairly soft metal, nickel wears off rather quickly with frequent polishing. Chromium wasn't used for plating on automobile trim until introduced by Oldsmobile in 1927, once chemists figured out how to plate it onto steel.
Now, how to "tarnish" model car "brass" trim, such as your Model T kit: Unfortunately, this is a job that calls for at least a basic airbrush, as in my experience, it's best done with a thin "wash" made from Testors "Rubber" (#1183TT) which is a flat brown-black color, in the little square bottles. I put a small amount of this in an airbrush color jar, then add enough lacquer thinner to make a thin wash. The lacquer thinner thins the paint to about the consistency of ink, which means too thin to apply by paintbrush. However, when airbrushed lightly on those "gold-toned" plated model car parts (your brass trim), it gives that brownish cast that looks very much like tarnished brass, along with a very dull, flat finish. With a bit of practice on the sprues, you can vary this shading to get the degree of tarnish you like, and unless you go overboard with it, the bright plating will still show through, albeit toned, or dulled down greatly.
If you, or anyone has ever seen the second issue of the old AMT 1907 Thomas Flyer "New York to Paris Race" winning car, with photographs of the built model on the box art, I am the person who built that model for Lesney-AMT back in 1979. The brass trim on that model is tarnished as I described, and it did come out surprisingly realistic.