[This post contains major SPOILERS for Star Wars Special: C-3P0.]
Even before Star Wars: The Force Awakens hit theaters, there was discussion about why C-3P0 had a red arm. Tidbits concerning the red appendage were dropped, but fans were told that they'd have to wait for an upcoming comic about Threepio to get the full story. Fortunately, that wait is over.
While it had previously been revealed that C-3P0 took his red arm in memory of a fallen comrade, the circumstances surrounding that droid's demise (and what happened to C-3P0's own arm) were shrouded in mystery. Now that Star Wars Special: C-3P0 is on the stands, though, the shroud has been lifted and we can finally find out where that arm came from.
Star Wars Special: C-3P0 tells the story of C-3P0, obviously, and a First Order protocol droid named OMRI that had previously been taken prisoner by the Resistance. OMRI may know the location of Admiral Ackbar, who had been taken prisoner by the First Order, but obviously is not keen to divulge this information. The ship they are traveling on crashes at the beginning of the issue, and the pair (part of a group of droids who survived the crash) head out into the hostile alien landscape in hopes of finding the ship's captain and crew. As you might guess, things don't go well.
Threepio's original arm is ripped off by a sea creature, and he and OMRI narrowly escape as another droid sacrifices himself to fight more creatures off. The crew the droids hoped to find is dead as well. Hiding in a cave to shelter themselves from corrosive acid rain, OMRI decides to tell C-3P0 Ackbar's location because he believes that he is facing his demise. OMRI then heads out into the rain to activate the distress beacon on a downed TIE Fighter in hopes that someone will come and rescue C-3P0; as he is pelted by the acid, the black coating painted onto him by the First Order begins dissolving to reveal a red base coat underneath.
Rescued by Poe Dameron and BB-8, C-3P0 retrieves an arm from what's left of OMRI's body. When Dameron comments that it looks like Threepio needs an arm, he mentions that he already has one. The arm is worn in remembrance of OMRI's bravery and sacrifice, as we all saw in The Force Awakens when C-3P0 worried that Han Solo didn't recognize him.
The story makes for a good droid-centric tale, and it's perfectly believable that C-3P0 would not only be on a ship that crashed but that he would also have his arm ripped off and somehow manage to survive all of it. Some fans may feel a bit let down by the tale after all of the build-up for it, though, especially since the mystery of the red arm was referenced multiple times by the cast and crew in addition to being pointed out on the big screen. Of course, it remains to be seen how long the red arm will last or if Threepio will go back to solid gold before the end of the current trilogy.
Star Wars: The Force Awakens is now available on Blu-ray and DVD. Rogue One: A Star Wars Story opens in theaters on December 16, 2016, followed by Star Wars: Episode VIII on December 15, 2017, the Han Solo Star Wars Anthology film on May 25, 2018, Star Wars: Episode IX in 2019, followed by the third Star Wars Anthology film in 2020.