One of the primary reasons we refer to the Hair shader as an approximation, rather than a simulation, is that it is not actually simulating every individual strand of hair, nor is it simulating perfectly accurate lighting behavior. In the real world, as light bounces off of and transmits through follicles of hair, it will often encounter other strands of hair, repeating the same process potentially many times in succession. At this time there is no computing system capable of accurately producing the effects of such a simulation in realtime.
Nevertheless, the way in which light scatters through a body of hair is still crucial to how realistic that hair appears to be, in a game just as much as in the real world. To control this, the Hair shader provides the Scatter property, which replaces Metallic on your primary shader node and is limited to values between 0.0 and 1.0. Scatter controls how much light passes through the entire body of your character's hair, as if it were a single surface.
An important point to note about Scatter is that it will tend to be higher for lighter colored hair and darker for darker hair. This follows the physical rules of the natural world, as darker hair will tend to absorb more light. In practical terms, if you are trying to create a blonde character, you will find that changing the diffuse texture or color is not enough; you will also have to increase the Scatter value as well.