Kate is wearing a very unique updo hair style. The hair is parted on the far left side and pulled up in a bun.
Use of different colors make this updo style stand out. She may be wearing some sort of hair extension or clip-in hairpiece that is different colors. Or she could have had her hair actually colored.
That is the beauty of hair extensions and hairpieces--no one can really tell if it is your own hair or not.