Why does my son have light brown hair and my daughter has dark?

My husband and i cant seem to find out why… my sister is light with light hair so maybe .. sometimes people tell me whos kid is that? Like bitch hes mine and my husbands. Its sooo annoying like a kid cant look different and think its somebody elses kid .. so fucked up . I felt sick to my stomach when my husbands family was saying tell me who he looks like cuz we all have dark hair. My daughter has dark brown hair ..