C3PO was originally silver, but when he came to be owned by Senator Amidala, he was given a golden body. After losing his leg, he was given a temporary silver leg replacement. Due to the fall of the Republic, he never received a new golden leg. Watch all of the films and you'll see the transition.
C3PO's leg was silver long before the prequels were a twinkle in George Lucas' eye.
Also they're not cannon and anyone who says different hates Star Wars.