It's always seemed reasonably obvious to me that hair color isn't an all-or-nothing thing, but then again I've owned dogs.
We tend to think of our hair as being all one single color, but really, just like most mammals, our hair comes in several different colors. I personally have curiously metallic silver hairs shot throughout my head hair (and no, I'm not just going gray, I've had them since I was a child), but they're so sparse in number that my dark brown hair conceals them almost all the time, despite their vibrant shine when the light catches them properly.
As for the brown hair, it comes in several shades, all dark, but with distinct differences. Some are glossier than the others, others are almost pure matte, and the middling textured hairs are for some reason the darkest of the three main types.
Then there's the much darker and coarser body hairs, and among them I get the occasional wirey silver variant, and even rare stubby "red" stragglers and pale "white" ghosts.
And of course separate from all of that is the colorizing effects of sun exposure or salt tinging that occur when I spend time at the beach.
Really though, ever since I was little I realized my hair wasn't any single color. Just like my dogs with their multiple coats and multiple colors within each coat. Look at any well-furred mammal, really - none of them has merely one color of hair!