Why C-3PO Had Red Arm in 'Star Wars: The Force Awakens'
— -- In the Marvel comic "Star Wars: C-3PO" No. 1, the writers helped fans understand why their favorite protocol droid didn't have all his matching limbs in "The Force Awakens" late last year.
Spoilers ahead if you plan on reading the comic!
The issue tells the tale of how a droid sacrificed itself for the Resistance, according to Gizmodo's recap of the comic. The mission is to find information on where Admiral Ackbar is being held prisoner by the First Order and also how they can save Mon Calamari from being executed.
The group is traveling with a protocol droid Omri, who was loyal to the First Order before being capture by the Resistance.
Omri becomes a lightning rod to question the relationship between droids and their masters. He doesn't necessarily agree with how much power the masters have, like cleaning the droids memories after completing missions. He doesn't like how C-3PO and other droids just follow orders without hesitation.
As the mission progresses, other droids in the group get destroyed and Omri and C-3PO begin to wonder what their pasts were like and how many times they have been wiped clean, because they can barely remember.
C-3PO eventually loses his arm and Omri sacrifices himself, but not before giving C-3PO the information his unit has been looking for. Omri walks out into certain death to activate a beacon and save his once-enemy. Acid peels away at Omri and reveals he used to be red.
In honor of his fallen comrade, C-3PO takes one of his arms, and there you have it!