The rifles that interest me are not the fancy carved and engraved style, which would cost three to four times the price of the most basic rifles being made, but the rifles of the small subsistence farmers and working men. I'm thinking these most often would have been white, but how bright?
The problem with this thinking is that it is not based in fact. A rifle would have been expensive. Period. There was no sense in getting a "plain" rifle, especially since a good gunsmith could carve, or even engrave (to an extent) a gun pretty quickly and easily. For an experienced gunsmith, carving does not take very long... very little time, actually. It would not be difficult, nor expensive to do. It was expected. This changed in the 19th century, as styles changed (unfortunately...), and as economics changed. But the 18th century "Po boy" rifle I think is basically a fantasy.
There are "relatively plain" rifles (or rifle-type guns) of the late 18th century. My opinion is that to a great extent, this may have been pretty much the standard level gun. But these existing guns still have all their normal hardware, and still have at least some carving in the normal places. Fore end mouldings, "teardrops", some kind of carving behind the breech tang.... Basically only omitting the carving around the cheekpiece.
As for barrels (and locks) being left white, that was pretty much S.O.P. Charcoal blueing was done, but not very commonly. Purposeful browning in the late 18th century seems to be pretty well limited to fine grade guns with Damascus/twist barrels, the browning showing off the pattern. I believe that for the most part, existing 18th century American rifles with browned barrels got that way simply from use (and abuse). It doesn't take very long for a rifle barrel to start turning brown. All I have to do is handle one, and it rusts up in short order. I will have a pretty heavily patinated rifle barrel before I'm even finished making the rifle! Without doing anything but scrubbing off the heaviest, reddest rust.
Keeping the barrel bright would actually require maintenance. As I recall, British soldiers were issued brick dust to keep their barrels scrubbed bright and shiny. A hunter may have desired the barrel to go brown (as evidenced by the above PA Gazette article), but I am doubting that it was really done in the 18th century much on purpose by the gunsmith. I may be wrong on that, but that's how it seems to me.