That's great advice.
I don't have the same issue with being half white/black, but I have a little bit of insight/experience with critical grandparents.
I'm basically latina, btw. I have some native american, and some Italian, too. My hair is very "ethnic," LOL, but I have alot of cousins with very similiar hair, and one cousin with even curlier hair, so no real criticisms on my hair from the side of the family that has the curlies; the OTHER side is a different story for a different day...
Anyway, when I was a child through my teenage years, my grandfather used to always criticize me about random things about my appearance. Not my hair, just things about my face. Once it was a small mole I had on my eyelid. Once it was my facial hair (no joke; like most women, I wax my brows, chin and lip, but when your a kid, you don't really do those things); another time it was my facial pores (not kidding). He would point out if I had a pimple or a cold sore. It was infuriating, it was embarassing, and it was kind of demoralizing. I think I would've preferred if he criticized my hair, to be honest.
I'm 40 years old now, and I still remember how that felt. And you know, he still does it. He's 86 now and a bit senile. I can't get mad, but I don't like it.
The point I wanted to make is, sometimes its a generational thing. To be that way. They fixate on something to criticize. I don't think my grandfather was trying to be mean, it was (in a sad way) his way of trying to relate to a younger generation. It was the wrong way, but he didn't know how to talk to his grandkids. I watch him now, and he doesn't know how to talk to his great grandkids either.
With my grandfather, he fixated on facial features; with your grandmother, it is hair. I think they fixate on the things they're insecure about in themselves.
I know it doesn't help to know that; I was always insecure about my face - I don't think I'm especially pretty, and to have someone always picking it apart is not an enjoyable experience. I imagine its the same when your grandmother has a go at you over your hair.
I don't have any real advice about how to handle it; I think Kat's advice is great.
I just thought I'd share a similar story hoping it helps shed some light on why your gran might be doing what she is. She may not be aware she's being as hurtful as she is; I don't think my grandfather ever was.