The melanocytes that migrate into the hair bulbs inject their pigment directly into the developing hair shaft. Their activity tends to be highest as the point of the new hair is first being formed, which is probably why many reddish animal hairs are darker at the tip than at the base. Once the hair is visible above the skin, the color of that part of the hair is fixed. In other words, if hair turned white overnight, you wouldn't see it the next morning, but only as white appearing gradually at the base of the growing hair, just like dark hair showing at the base of the hairs as a bleach job is growing out. The change is observed to take place very rapidly along the hair, and on a given animal affects only certain kinds of hair.
Tabby cats have solid black hairs in the dark stripes and banded ones in the light stripes.
Wolves, coyotes, rabbits, mice, and most other wild animals have a high percentage of banded hair. If you don't have a pet, try your parka ruff. You may be surprised at the complex pattern the melanocytes can produce. Related articles are available on black vs. brown or yellow pigmentation , dilute pigmentation , Siamese coloring in cats , and white markings in domesticated animals .