When analyzing Jack's hair as a symbol, I suggest that you also consider doing so in reference to the black cap as well:
"Inside the floating cloak he was tall, thin, and bony; and his hair was red beneath the black cap" (20).
When Golding first introduces Jack, the important detail about his hair is that is under the black cap. The black cap, a visual symbol of his connection to the choir, and through it, civilization, keeps Jack's hair neat and controlled; his hair is as orderly as his role as head choir boy. In the beginning, Jack's neatly defined hair suggests that he is a follower of the rules.
Later, as the novel progresses, and Jack starts wearing the painted mask for hunting, his red hair becomes one of his only identifiable characteristics. He has traded the black choir cloak for a frightening mask, and his once neat hair is also loose and wild, reflecting his transition from model choir boy to desperate savage.