* Identical twins (monozygotic) start from a single fertilized egg that splits. They have nearly identical DNA, so they usually have the same hair color, but slight variations can occur due to environmental factors or random genetic changes during development.
* Fraternal twins (dizygotic) develop from two separate eggs fertilized by two different sperm. They are no more genetically similar than regular siblings, so they can have different hair colors (along with other differences like eye color, height, etc.).