Darklands Puzzle #23

The path is blocked by a grim iron door. Carved overhead are the words:
EACH SPEAKS EITHER WHOLLY TRUE OR ALL LIES.
TO PASS, TOUCH THE ONE THAT MUST SPEAK TRULY.
Embossed on the door are three metal faces: gold, silver and copper.
Two of the faces speak.
Silver: "At least one of us three is a liar."
Copper: "Press gold to open the door."
Which is the correct choice? A mistake could (will. ed.) release a trap! You think carefully, then touch...
...the gold face.
...the silver face.
...the copper face.

(Source: Darklands cluebook by MicroProse Software)

If Silver is lying then Silver is not lying, because Silver makes for at least one liar. This is a contradiction. Therefore Silver must tell the truth and one should touch the Silver face.