If two parents have dark hair, the child will most likely have dark hair.
If one has light and the other dark, the child has a 75% chance of having dark hair or a dark-dominated mix because the gene for dark hair is dominant.
If two parents have light hair, the child will most likely have light hair.
Of course, it gets a lot more complicated than this, the genes of the child's grandparents and great grandparents could be carried by the parents as well. So if the parents have dark hair but they each have a parent with light hair, the light hair gene will be carried by them and could be passed on to the child. That's how it is with genetic disorders, such as Systic Fibrosis or Sickle Cell Anemia.