Darklands Puzzle #9

A door blocks the path. Its inscription reads:
FACES ALWAYS LIE OR ALWAYS SPEAK TRUTH.
TOUCH ONE KNOB TO OPEN THE DOOR.
On the wall are six numbered faces, who speak:
One: "Copper cannot open the door."
Two: "Face One speaks the truth. Face Four speaks truth. Gold cannot open the door."
Three: "All odd-numbered faces speak truth."
Four: "If Face Three lies, silver opens the door."
Five: "Gold opens the door."
Six: "Black is white. Copper opens the door."
Upon the door are four metal knobs. You touch...
...the gold.
...the silver.
...the copper.
...the lead.

(Source: Darklands cluebook by MicroProse Software)

The first clue is a gimme: Six is claiming black is white and is therefore a liar. This means that the copper knob does not open the door. This means that One speaks the truth. In turn, Two must speak the truth, because all faces are perfectly uniform about lying or speaking the truth. Four, who has already been established as a truth-teller, says that, if Three lies, silver opens the door. Five is lying, meaning that Three's claim is a lie. Therefore the silver knob opens the door.