A new Marvel Comic one-shot,C-3PO: The Phantom Limb, finally answers the question unequivocally.
But… were not answering it here.
The question were going to tackle avoids spoilers: NothowC-3PO got his red arm, butwhy.

Credit: Marvel
Joining him are five other droids:
the prisoner, OMRI, a mantis-eyedDeath Star droid.
PZ-99, atowering blue security droid, serving the Resistance.
CO-34, a construction droid who can transform into useful devices, but only recites its own name.

Marvel
2-Med-2 (or Toomedtoo), a medical droid.
VL-40, a smaller security droid just an eye and guns on treads.
A young monk who witnesses the accident sets out to explore the lives of the victims.

Was there significance in their demise, a purpose to their existence and sudden ends?
Or is the universe random and meaningless?
Even memory is not necessary for love.
C-3PO:It clearly bothers you, OMRI, that our memories are in the hands of our makers.
Youve raised the subject more than once.
He goes on…
OMRI:I think that added awareness causes us to question.
Flashes of past events were they grand events, or nothing to speak of?
How important have I been?
These questions nag at me.
You, on the other hand, See-Threepio, blindly and eagerly obey your orders.
I assume you recall nothing.
C-3PO:Thats not entirely true.
I like to think that Robinson and Harris are exploring bigger existential questions.
What is good or evil?
Are we incidental and alone, or is there a larger, unseen power guiding us?
The Phantom Limbasks these same questions in a fresh, funny, and compelling way.
Why does C-3PO have a red arm?
The same goes for C-3POs red arm.
Knowing that shouldnt diminish your interest in the story.
We tell stories to make ourselves happy.
Theres meaning in that, too.
For moreStar Warsphilosophizing and whatnot, follow@Breznican.