Spoilers for Marvel Comics' C-3PO Special Ahead!
The Marvel Comics C-3PO Special has finally hit shelves today, and it reveals C-3PO's biggest mystery: How he got that red arm he was sporting through most of Star Wars: The Force Awakens. Major spoilers ahead!
The tale takes place an unspecified amount of time ahead of the film, as C-3PO and a squad of assorted droids are the only survivors of a crash. Their mission is to get another droid, identified as Omri, back to the Resistance, as he has intelligence on the First Order - specifically, where they're holding Admiral Ackbar.
The droids go on a bit of a journey, trying to find a crashed TIE fighter so that they can use the transponder to get a message out for a rescue. The native creatures of the planet, however, have other ideas, and the droids have problems navigating the journey. One by one, the droids are destroyed, until it's only Threepio and Omri left.
The two have a tête-à-tête throughout, as they both talk about how they've had their memories wiped repeatedly in their lives. "We could've been allies before," they say to each other. And who knows, maybe they were? Threepio reveals that his memory wipes have never been quite complete - he "dreams" in a sense, remembering things from another age - when he was built by Anakin (though he doesn't remember that name) and a part of the Clone Wars (he even remembered his head being put on a battle droid's body temporarily).
As the two journey, Threepio's arm is torn off by a native creature. But that's only half the story - eventually, they reach the TIE, though they're slowed up by acid rains. Omri sacrifices himself for Threepio. The two had come to terms in a very "Heroes On Both Sides" (The Clone Wars) or "The Forgotten Droid" (Star Wars Rebels) method. As Omri walks through the acid rain, his arm's coloration is stripped away, revealing a red one - it then falls off at the joint. As Threepio makes his way back to the Resistance, he keeps the arm.
When he puts the arm on his own body, he vows to keep it red "at least for awhile" in remembrance of his fallen droid friends.
At the end of Star Wars: The Force Awakens, Threepio's arm is gold once more, so apparently the time had expired, and with everything that had happened, who could blame him? Ultimately, with Luke Skywalker possibly coming back soon, C-3PO probably just wanted the gold arm so that he'd have no trouble recognizing him, after his similar statements to Han Solo and all.
Read through the issue yourself online here, or head to your comic shop to check it out in print. The issue also reveals a story element that was in the script to The Force Awakens, but not in the final film, showing Threepio's new status in the Resistance as a droid leader.