Symbolism, Imagery, Allegory
Luscious Locks
Hair (or lack thereof) is a pretty big part of people's appearances, and in some cases, their identities. Sadima learns this the hard way when Somiss orders her to cut her hair, because he's feeling paranoid about being found:
Somiss reached out and pulled a handful of her hair. Sadima stood wide-eyed, like a kitten that comes upon a snake in the grass. "Anyone would remember this," Somiss said, tugging hard enough to make her wince. "Cut it short," he said. He glanced at Franklin. "See that she does it." (47.34)
This freaks Sadima out, obviously. She realizes that she doesn't want to cut her hair, even though it's a distinctive color of red and pretty long. Her compromise is to wear a "dull gray cap" (49.1) that Rinka gives her, which completely hides her hair, and also makes her look a little boyish.
We're thinking that Sadima's way of thinking about her hair symbolizes the way she thinks about herself in general: her hair stands out in a crowd, and so does she. She tries to accept that she's just different from most folks, and she looks for compromises (such as wearing a cap rather than cutting her hair). Maybe someday she'll stop conforming to what other people want from her (or her hair, as the case may be).
Dudely Dreads
Hahp's hair also symbolizes something about him: his living conditions. Since he and the other boys go for basically a year without being able to bathe (there's water, but no soap), their hair mats up into something like dreadlocks. When Hahp finally figures out how to manifest soap, he takes his first real bath:
The water was icy and the rough washrag hurt as I scrubbed, rinsing the cloth clean over and over. It was harder to wash my hair, and I had no comb, so dragging my fingers through the tangles was painful. And none of it mattered. It was glorious to wash my stink down the drain. (50.18)
It's not like Hahp wanted to go without a proper bath for a year and get his hair all matted up, but the wizards pretty much forced him to. So his newly-clean hair represents him taking back control over part of his life… it may be a small part, but it's better than nothing.