Pure speculation, but I think 3-PO's red arm signifies he is part of the First Order. The FO does a lot of red, especially red markings on the left side of things (the Special Forces TIE, Phasma's cloak) so I think this is in keeping with that. How/why/if he really is of course no idea, and the image of him yesterday with R2 has him among Resistance people, so who's to say. But it would be an interesting way of utilizing a character who has no function in the plot anymore. He could be a spy/plant.