Readily available is "Prussian Blue." Permatex makes it, and every auto parts store in the US carries it. Get a package of throwaway acid brushes to apply it to the barreled action when you get a tube of Prussian Blue. Just squeeze out a pea-sized squirt from the tube, and apply it with the brush.
Lipstick, shoe polish, and other such markers can be made to work, but the Prussian Blue works quite well and shows up very nicely on the stock wood. It is also easy to clean off the metal, whereas shoe polish and particularly lipstick are the devil to get off the action (and to keep off your shirt, which could be critical if your spouse does the laundry).
Clemson