Silver coloration (was polishing skills)

May we digress? About that grey color … I recently viewed an
exhibit of Thracian gold and silver. Much … but not all …
of the silver is an unusual color, kind of darkish purplish
silverish greyish. A friend (who may be lurking!!) maintains
the pieces might be a variety of niello, while I, in my
confident IN- experience, wonder if the color is due to being
buried in a tomb for two or three, and perhaps as much as four,
thousand years. (yes, thousand!) Has anyone seen this deep
coloration/discoloration? Are there any other theories or
explanations for this color? Does anyone know how to
deliberately achieve this color, short of niello or niello-like