We finally know why C-3PO has a red arm in 'Star Wars: The Force Awakens'
Marvel has released the comic book "Star Wars: C-3PO Number 1" and inside we find out the origin of the droid's striking red arm he was sporting in the movie.
If you plan to buy the comic and don't want to be spoiled, we suggest you stop reading now.
Okay, for the rest of you, this is how it happened.
In the comic, C-3PO is with other Resistance droids who have on their ship a captured First Order droid named Omri when they crash-land on a planet. 3PO and Omri end up being the only surviving droids, though 3PO has lost an arm in the crash. They talk about the role of droids in the galaxy, and 3PO tells Omri memories from events in the prequel films. Then Omri decides to sacrifice itself in order to save 3PO.
3PO takes Omri's arm in tribute of the unselfish act of the First Order droid.
According to the comic's website on Marvel, this is the only place where the red arm backstory will be explored, so don't expect much about it in the movies.