In Star Wars: The Force Awakens, C-3PO's left arm is colored red to symbolize that it was given to him by another droid as a replacement after he was separated from his original gold arm while on a mission.
The red arm serves as a unique identifier for C-3PO and helps to distinguish him from other droids in the movie.
The color choice also adds an element of storytelling, as it raises questions about the circumstances that led to the replacement arm and hints at possible adventures that C-3PO may have had between the events of the original trilogy and The Force Awakens.