Most experts say there is no way to make facial hair grow in darker, but there are a few tips you can try to make it appear somewhat darker.
Shave frequently, if your skin can tolerate it. Hair has a tendency to be darker in color near the root. Your growing-out stubble will appear darker. The "5 o'clock shadow" will look darker than several days' growth because hairs are thinner and lighter at the ends. This "scruffy" look works for many Hollywood stars, and it might work for you.
Use facial hair dyes. If you have a good start on a beard but the hairs are growing in blond or red and not showing up well enough to suit you, consider facial hair dyes. Be sure to select a product created specifically for use on the face. A couple of popular brands include Just for Men or Pialan. Match the color to the color of hair on your head. If you're bald, select a color close to what your natural color was. Follow directions on the package carefully. If the color doesn't work for you, shave it off and try another shade when you have new growth.
Take vitamins. Overall good health contributes to healthy facial hair. Daily supplements recommended to promote strong hair follicles and healthy hair growth include 50 mg each of vitamin B-6 and bitamin B-complex, 1 to 2 g of vitamin C and 400 to 800 IU daily of vitamin E.