Possible explanation for Tailoring: Healers restore the body to it's default state, Heartrenders change the body away from ita default state. So Heartrenders can change facial features but not change hair or eye color, only a true Tailor can do it. This makes me wonder if Heartrenders can produce insulin for diabetics or fix other autoimmune diseases🤔
You've managed to find that part of biology I've always hated- metabolism.
If I understand it correctly, Grisha can make organ produce substances, but immediately, not "reprogram" the organ permanently.
... Most Grisha Corporalki focused on the body—to kill or to cure—but Nina ... slowed heartbeats, eased breathing, relaxed muscles. She had a lucrative side business as a Tailor, seeing to the wrinkles and jowls of the wealthy Kerch, but her chief source of income came from altering moods. People came to her lonely, grieving, sad for no reason, and left buoyed, their anxieties eased. The effect didn’t last long ... Nina claimed it had something to do with glands ... “You make him feel better, ease his woe and all that … but could you compel him to do something? Maybe make him forget his wife?” “Alter the pathways in his mind? Don’t be absurd.” “The brain is just another organ,” Kaz said, quoting Van Eck. “Yes, but it’s an incredibly complex one. Controlling or altering another person’s thoughts … well, it’s not like lowering a pulse rate or releasing a chemical to improve someone’s mood. There are too many variables. No Grisha is capable of it.” Yet, Kaz amended. “So you treat the symptom, not the cause.”
Six of Crows- Chapter 5
But then again, this is ordinary teen Heartrender with unfinished education. We know nothing about old, highly specialized, talented, amplified Healers. Sure, brain's off limits- it has too many functions- but what about other stuff-producing organs?
Apparently, changing hair-colour is only about transporting pigment. Eyes probably work the same, but take more time- you don't want to fuck up your eyesight to look prettier:
... ordinarily Nina would have used pigments to alter her own hair color, but there simply wasn’t time. Nina bled the girl’s bright red color directly from the strands of her hair into Nina’s own, leaving the poor Kaelish with a mop of white waves that looked vaguely rusty in places, and Nina with hair that wasn’t quite Kaelish red. Nina’s eyes were green and not blue, but that kind of tailoring couldn’t be rushed, so they’d have to do.
Six of Crows- Chapter 28
My best guess on difference between a true Tailor and dabbling Corporalnik is something akin to aesthetics surgeon and dermatologist vs. general doctor. Plus Tailors have a sense for chemistry, so they should be able to draw from non-organic, or possibly toxic sources.