It changes colour based on your current hit/persistent charge amount, so does the colour of his specials
I just ran an easy quest with Aegon, finishing various fights agains varied opponent classes with light, medium, heavy, Sp1, Sp2. My combo never got over fifteen and every victory animation had a blue arm. I guess it depends on whatever combo bonus you're up to - does anyone know when the arm colour actually changes from blue to gold to red?
I still haven’t figured out which arm it is. It always changes depending on the side he‘s facing.