"White lead" is lead carbonate powder in a linseed oil carrier. It's easier to see than lead oxide which is used to make "red lead". It was once used to make white paint before the tree huggers got their frilly undies in a wad about anything containing lead. Artists' supply stores sell a modern equivalent pigment that's supposedly lead free. Prussian blue is a grease based dark blue dye that machinists use to check the fit of parts with, and it hasn't been banned- - - -yet. A 2 ounce tube of Prussian Blue is probably a lifetime supply for fitting rear end gears.

"Any of the above" can be used to check tooth contact patterns. Unless a set of used gears had the tooth contact checked and photographed before they were removed from their original housing, the chances of getting a noise-free installation into a different housing are "slim" and "none".
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!