There are two ways in which you can answer this question.
On the literal level, Jack's hair gets lighter over the course of the book because it has been exposed to the sun. The sun naturally breaks down the color in a person's hair (follow the "thetech.org link for more). Jack would not have been exposed to nearly so much sun in England so his hair would not have been lightened there.
Metaphorically, the change in Jack's hair color is part of the overall change that the island has caused in him. He has been becoming more and more different than he was back in civilization. As he changes inside, the change is also reflected in metaphorical terms in the changes in his appearance.
Jack's hair grows longer and turned into a sandy color shows that he has stayed on the island for a long time. No longer staying in his cozy home in England and without parental care,his doesn't cut his hair by his parents .This shows Jack has been away from the civilized adul world.Moreover,long hair symbolizes untidiness and savages.