I did try the mirror, but they refuse. I just tried it with my character and it works. It's not my character's portrait that needs updating though, but I swapped armor, clicked the mirror, didn't do anything except click confirm and it updates.
I've only dyed my character's armor, and the portrait is reflecting the colors I dyed it. The default color was some weird green and brown color. I dyed it black and red, and it's definitely black and red in the portrait.
