Why was 3PO's arm red?
Good question. I figured he lost the arm somehow, and the only replacement they had on hand was red. Seems to fit in with the way things work in Star Wars, but it was odd how they pointed it out, never explained it, and had his arm gold again by the end of the film.
On the other hand, maybe he was turning to the dark side, and evil droids are all red. I want to see that movie!
He forgot to let the wookie win.