Why is the human body hair not uniformly colored?
The hairs you mention are also called "androgenic hairs", meaning their growth and pigmentation is influenced by androgens. These include pubic hair, the hairs on the breast and shoulders (almost exclusively for men) and the beard.

It seems, that these hair bulbs have different sensibilities (number and expression of androgen receptors) so they react differently to androgen levels. This includes balding, pigmentation, growth and so on.

These articles should be a good starting point, if you want to dive deeper into the topic:


Androgen actions on the human hair follicle: perspectives.
Mechanism of action of androgen in human hair follicle.