I would think that graying is a visible effect of senescence (i.e. aging), and that rather than being an advantageous trait that has been evolutionary selected for, is non-adaptive, and that there is simply too much of a cost to continuously produce the pigments in hair. So then it becomes a question of why hair is colored in the first place, and to that, it probably co-evolved as a secondary sexual trait to signal health in individuals that were of a reproducing age, at least in humans (in other animals it acts as camouflage, as a means of mimicry, insulation, protection, defense, etc. but most/all of this ancestral utility of hair has been lost).
The idea that gray-haired humans have reproductive advantage could be true; but then that gets more into (1) what exactly 'choosy females' choose, i.e. if there is something like a density dependent selection acting on a population, the choosy females might, for example, be attracted to the rarest hair color. But also, (2) that hair itself needs to gray prior to age before reproductive isolation starts to occur (i.e. sterility, infertility; among other barriers), otherwise reproduction will not occur among gray-haired individuals. Consequently, if choosy females choose gray-hair, there might be more pairings/matings between gray-haired individuals, but fewer offspring.
So the things I've touched on are:
- Non-adaptive traits
- Vestigial Traits
- Selection (and types of selection)
- Sexual Selection
- Reproductive Isolation
What I haven't talked about is a social paradigm (culture) that could have evolved on top of the aforementioned to make it so that gray-haired individuals are more fit (Which is sort of what your original question is grasping at.) So, I would say, try to prove to yourself that someone with gray hair would have higher fitness than someone with any other hair color. What does having gray hair indicate? Is your theory actually reflective of real-world phenomena?