Like Dave, mine was in a race between the gray and the MIA. I remember when the lady at the driver's license place asked "Weight?" and added 10 lbs that I had conveniently forgotten, and "Hair color?" and I said 'Brown." and she mis-typed "Gray". Now, there is not much wiggle room. It's all just snow white. Oh well, at least there's enough to be able to tell. I DO recall that when there was a mix, the silver threads were thicker and straighter that the brown ones, and they grew faster.
Jim