[Skoob]

C-3PO's Red Arm In Force Awakens Explained (c/o CinemaBlend.com)

Quote:

C-3PO is uncharacteristically quiet when it comes to discussing his salvaged arm; it is a memento of another droid's sacrifice....the protocol droid will be the focus of a canon comic book piece. According to ComicBook.com, this comic one-shot will apparently explain how the red arm came to be, and presumably will pay homage to the droid who sacrificed its equipment so that 3PO can continue to be helpful to the resistance.

I just find it annoying, like when a buddy with a junker vehicle replaced a side panel with a matching part from a different-colored car. Used to bug him about it until he repainted it with household latex white...