I'm reasonably sure that there is a degree of variation in hair colour over time that is consistent with bleach jobs. I'm hardly an expert though. However, for the sake of arguement, in HYA, Buffy is naturally a darkish blonde and customarily had her hair bleached lighter before she got to Sunnydale. notBuffy didn't bother with bleach and rarely bothered with hair cuts, so she had darker and longer hair than in canon.
D for Drakensis
You're only young once, but immaturity is forever.
D for Drakensis
You're only young once, but immaturity is forever.