In Star Wars: The Force Awakens, We run into C-3PO again. For some reason, he has a red left arm. However, the situation/reasons behind how he got this red arm is glossed over (even by the character himself if memory serves me correctly).
Is it just a fresh coat of paint on his old arm? A salvaged part from another (presumably destroyed) Protocol droid? A newly manufactured arm that is able to perform more/different functions?