If Both Parents Have Brown Hair Because Both Carry Two

If both parents have brown hair because both carry two alleles for brown hair, their children will also have brown hair, but it might be a different shade of brown than either parent. Why is this? Isn't brown brown?

A) The effect of an allele changes depending how old the parents are.
B) People's hair often gets darker as they get older.
C) Several different genes are involved in hair color, and those might differ between the parents and therefore their children.
D) Traits tend to vary quite a bit between siblings and parents, so it's unlikely their hair color would be identical.
E) What one person describes as "brown" might not be exactly the same as what another person says is "brown."