In the fleeting glimpses we've seen of him from Star Wars: The Force Awakens promotional materials, it's clear that C-3PO has undergone some changes over the 30 years since Return of the Jedi.
Chief among them: the fussy protocol droid now has a red arm.
It's not the first time C-3PO has been seen with different colored parts. He was first constructed without armor plating at all, then later received used and tarnished coverings. Eventually, he got nicer, gold plating, but one of his lower legs was silver. Different explanations have been given over the years for that mysterious limb, but the bottom line is that it was a decision by the filmmakers to give C-3PO "scars," as in layers of backstory.
That was likely J.J. Abrams' motivation for giving him a red arm for The Force Awakens as well. However, this time, the Lucasfilm story group is prepared to offer a canonical explanation for how and why he wound up with a replacement part that doesn't match the rest of his body.
Marvel Comics is releasing a one-shot comic book in December titled Star Wars Special: C-3PO #1. An official part of the "Journey to The Force Awakens" books, comics and games, the special issue is being crafted by an auspicious team: James Robinson and Tony Harris. Comic book fans may remember Robinson and Harris as the duo behind a critically-acclaimed eight-year run on Starman back in the late 1990s.
Marvel promises that the issue will reveal "the shocking truth behind how C-3PO lost his original arm and the exciting origins of how he gained a new one." The issue is being viewed as a prequel to the new movie — or at least one crimson-colored piece of it.
Star Wars Special: C-3PO #1 will be available sometime this December.