I too got a sense that the question pertained to historical correctness and must agree with TG and Mike. "Plains" type rifles were fitted with either iron or brass furniture. Either one works.
BigMike, if you're trying to make brass look like iron, I think you're going to be dissappointed. Whatever finish you apply to brass will wear off to a certain degree, especially on the trigger gaurd and buttplate, and a yellow/red brass undertone will show through somewhat. In the end, brass and iron are two different animals.
"Aged" brass looks nice and may be more what you're looking for. Like mentioned earlier, B/C super blue or aquafortis (or any acidic or caustic substance) will color brass to give different patinas. Bill