What causes the differences in physical characteristics like hair color among people?

I'm pretty sure that the physical characteristics among people are caused by traits. Dominant traits overpower recessive traits, so if the offspring receives a dominant trait and a recessive trait then the dominant trait will show. However, it is possible for the offspring to have recessive traits. You see if both parents have the recessive trait then their offspring could receive the recessive trait which allows it to show.

Dominant traits are shown with a capital letter, while recessive traits are shown through lower case letters.

For example: One parent has blonde (b) hair while the other parent has brown (B) hair. Their offspring could have BB hair which means that one kid would have brown hair. One kid might have the Bb trait, which still means his/her hair will be brown, and another kid could have the bb trait which means the offspring's hair would be blonde.