Dyeing a beard a temporary white is difficult with regular box dyes. They require bleach and multiple applications to lighten the follicle -- and the color is semi-permanent. Spray-on color is a safer and easier method. It provides full coverage on even the darkest of hair, and washes out when you want.
Cover your clothing with an old T-shirt or smock that you don't mind getting stained.
Pick up a quarter-sized amount of petroleum jelly and smooth it over the 2 inches of skin surrounding your beard to protect it from the color.
Hold a towel over your mouth with one hand to protect it from the spray. If possible, lay down and have someone else apply the spray.
Hold the spray can about 10 inches away from your beard. Press down on the nozzle while continuously moving the spray over your beard for an even coating. Stop spraying after the first coat.
Allow the first layer to dry. Apply another coat if the color is not solid enough.
Dampen a cloth with warm water and use it to wipe the petroleum jelly from your skin.