Excessive Sun Exposure
Facial hair is subject to the same physical/environmental factors that can influence body hair in general, including that of the face and head. A major factor governing the changing color of facial hair is excessive exposure to the sun. The sun's ultraviolet rays have a very penetrating effect on body hair, and excessive sun exposure can cause darker-colored facial hair to fade and become lighter. This is caused by the sun's effects on the melanin and pigment contained in each hair follicle.
Natural Aging Process
As a person ages, it is common for the pigment in his body hair, including facial hair, to lose some of its natural coloring and become lighter. Facial hair will gradually start to lighten, and eventually the hair will start to turn gray. Each hair follicle gradually starts to lose its ability to produce the same amounts of melanin and pigment--both of which cause the hair to lighten.
Certain prescription medications can have an effect on a person's facial hair. Certain antibiotics and some heart medications can affect the color of body hair, including facial hair. Many times these medications may stimulate increased melanin uptake, which often results in facial hair becoming darker.