I seek a realistic method, either by genetic change or other scientific method that doesn't require magic or future tech, to obtain naturally occuring two tone hair for a race of humanoids.
My idea is simple, as the hair grows in length, X happens that causes a natural change in color. Basically the higher the "Age" of the hair, the more it changes, either by exposure to air or what have you.
The issue is, I can't solve for X.
I'm already using another race that has cyanophores in their hair naturally from consuming creatures that contain them on their planet over time. Their effect is simply when their hair gets wet it slowly changes to a bright blue.
But for two tone hair? I'm stumped.
Is such a thing possible?
Please keep in mind their home planet doesn't need to be just like earth, I will adjust it to meet this possible requirement.
Also: I seek to explore the possibilities of how the hair could change color quickly enough to make the switch from say, brown to blonde occur quickly enough to allow the gradient change to only appear in around half an inch of the hair itself.