Well this actually used to be my profession many years ago lol
You say engraved, we just need to clarify that the lettering is incised (cut into the stone) and the type of stone. If it is white marble and the lettering is actually incised/ engraved, what we used to do was to just very roughly paint the lettering after first cleaning the gravestone, then rub over the whole inscription with a large piece of pumice stone to remove the excess. We used just a basic metallic paint with an undercoat.