As you can see, the swords turn BLACK, even though in one image you can see that it is dyed... but then will just abruptly turn black. I even tried removing the dye, but it just turns black even without dye. I haven't tried other weapons. I know on my regular character, the staff dyes and shows just fine... so far it only seems to impact companions.

EDIT:
Just wanted to mention that it stays black outside of the outfit station too, that's when I first noticed it. I equipped the dual swords to Bastian, dyed them, then exited the outfit station and noticed they were completely black. Went back into the outfit station and tried removing dye and it would briefly return to normal look, then switch back to being completely black.