My first Reo bought used is a copper vein Mini 2.1 that had already turned almost entirely black. There is no way to restore that original copper vein color itself that has darkened.
Options that I know of...
...learn to live with it like I have.
...strip all the original finish off back to tumbled raw aluminum and call it good.
...explore the possibility of strip to TRA and apply a new finish of your choice (via Reosmods, other sources, etc).
HTH