Some of the hairs that our cats shed are multicolored–that is, the individual hair has bands of different colors. I have seen this with other animals as well. This would seem to imply that the hair changes color as it grows.
Why does a hair change color as it grows?
Does this happen in all mammals?
If the hairs are changing colors as they grow, why do the cat’s markings stay the same over time?
The agouti gene produces banded hair by periodically changing the kind of pigment (eumelanin or a reddish pigment called phaeomelanin) produced by the melanosomes that are injecting pigment cells into the base of the hair. The change is observed to take place very rapidly along the hair, and on a given animal affects only certain kinds of hair. On bay horses, for instance, the coarsest hairs (mane, tail, lower legs and whiskers) are black, the softer body hairs are red-brown, and variable numbers of black-tipped red hairs appear along the back, sometimes producing shadings down the sides. Tabby cats have solid black hairs in the dark stripes and banded ones in the light stripes. Agouti coloring in domestic dogs ranges from fawn with a few black-tipped hairs (great Danes and boxers) to dogs with black guard hairs and a tan undercoat (my Shelties) to dogs with several bands per hair of black and very pale cream (Norwegian elkhounds). Wolves, coyotes, rabbits, mice, and most other wild animals have a high percentage of banded hair.
from here: http://www.gi.alaska.edu/ScienceForum/ASF8/873.html
The ancestor of the domestic cat for instance has what is called a ‘mackeral tabby’ coat - broad dark stripes and agouti banded greyish-brown fur in between, as with this fellow. The stripes act as ‘disruptive camouflage’, breaking up the visual image of the body in cover, while the agouti portion acts as ‘concealing camouflage’, making the animal blend in with the background.
My hair changes color in the summer and the winter, as does one of my brother’s. In the fall we’ve sometimes noticed that our ends are lighter than our roots. We’ve never noticed the opposite in the spring, but then, it’s not like we spend our time staring at each other’s hair.
Also, I don’t have many white hairs, but one of them went white recently: the end of it was still dark brown, the root was white. I found it quite intriguing, I’d never seen a hair in the process of turning white before!
(I don’t dye my hair, so it wasn’t a case of the white root showing, but of the white root growing)
My eyelashes are blonde at the roots and dark at the tips, and have been my whole life. Am I a tabby cat? 
I’d like to know this as well. We had three cats (sisters from the same litter) who had pale “frosted” tips to their fur hairs. It was most noticeable on the calico, who looked like she had a thin layer of flour dust over her, especially when standing next to our other, normally coloured calico. All three gradually became darker with age, but still had noticeable traces at 14 years of age. It seemed almost as if the pigment just evaporated from the tips of the hairs, as the frosting didn’t appear to affect any part of the hair other than the tips.
This could be due to sun exposure bleaching your hair. You would be comparing hair with several months of exposure to hair with only a few weeks.
