Both permanent and semi-permanent hair dyes are safe to use on a beard and effectively dye beard hair. Although temporary hair dye is safe for beard use, it has a tendency to run when wet and is best avoided.
Even though hair dye is safe for use on beards, there is always the possibility of an allergic reaction. Test any dye on a small area, and check for signs of skin irritation before using it on your entire beard. Keep all dyes away from your eyes and mucus membranes. Dyes formulated especially for beards provide another option for those who wish to color their facial hair.