Confusion can come from the fact that archeologists (who write the books) incorrectly call certain silvery-grey metal buttons simply Tombac buttons. In actuality, there are two basic types of Tombac: the color of one type is golden, the other is silvery-grey (or "white").
In Metallurgy dictionaries, the metal named Tombac is a form of brass, containing approximately 85% copper with 15% zinc. Simple Tombac's color is "golden" (distinctly more "orange" than yellow brass). The other version of Tombac, which we button-diggers are familiar with although most do not know its correct name, is called White-Tombac, because it is classified among what metallurgists call the "White Metals." White-Tombac differs from ordinary Tombac in that it contains a small percentage of Arsenic (which is a metal) along with the usual copper and zinc. Adding Arsenic to the molten copper-&-zinc alloy changes its color to silvery-grey. The Arsenic in the alloy may be the explanation for why White-Tombac buttons often come out of two centuries of burial with little or none of the patina-encrustation we typically see on dug buttons.
During World War Two, some Canada 5-cent coins were made of simple Tombac. Here is a photo to show you its color. The other photo (a 1998 Romania 100 Lei coin) shows what White-Tombac looks like when it is highly polished.
The confusion could be remedied if the late-1700s buttons were called White-Tombac buttons instead Tombac buttons. The DAACS document's section on "Button Metal Color," it says (but doesn't "do" in the charts there)... "Tombac buttons should be [classified as] white, unless there is evidence of gilding (goldplating)." See section 1.15 in the DAACS document.
http://www.daacs.org/wp-content/uploads/buttons.pdf