I'm not sure "generally speaking" applies to Northampton rifles - they're all over the board! Acer is right on the money with his observations, and any red coloring seen on Lehigh guns is in the finish alone. Red violin varnish could either be Dragon's blood, which is type of palm tree resin dissolved in alcohol and rubbed on as a french polish. It's very shiny, very red, and from what I've heard and seen somewhat on the fragile side of finishes, but it is beautiful! The other traditional red varnish is Madder lake pigment mulled into a slow drying oil varnish, then applied in many thin layers over the unstained or lightly stained maple. The photo below shows a contemporary pistol I stained with a VERY mild solution of aquafortis, sealed with seedlac (like shellac but darker), finished with madder pigment varnish, and subsequently rubbed back in the grip to contrast colors.
For non-red Northampton guns I've seen, their finish looks just like any other, which is safe to assume aquafortis was used.
-Eric
