Why did C3PO have a red arm?

C-3PO was completely covered in Gold plating in Episode III (Revenge of the Sith) as he was an Interpreter to Senators in the Galactic Senate. Somewhere between Episode III and Episode IV (A New Hope) C-3PO's left leg was implanted with a Bomb. But the leg was replaced by an unknown droid. But was replaced with a Silver leg as there was no gold parts available.