Whatever your reason for wanting to dye your hair green—St. Patrick's Day or Halloween, to root for your favorite sports team, or just for the heck of it—you probably you don't want your hair to stay that color forever. Food colorings are a great thing to use as a temporary dye, in part because they're not harsh and won't damage your hair.
Put on your gloves. As a general rule, anything that will dye your hair can dye your skin as well—and this rule certainly applies to food coloring.
Fill a shampoo bottle about a quarter of the way. Empty your bottle of green food coloring into it. Add about 1 tbsp. of water. Shake for about two minutes.
Cover your shoulders with an old towel to protect your clothing.
Apply the mixture to your hair a little at a time, working from the roots until all your hair is covered.
Leave the mixture on your hair for approximately 30 minutes, enough time to saturate your hair with the green dye. Adjust the time based on your hair color—lighter hair will need less time and darker hair will need more time.
Rinse the dye out with lukewarm water. Dry your hair with a clean towel.
Don't use Prell shampoo, as it will inhibit the dyeing process. However, if you are having trouble getting the color out once you've had your fill of green hair, Prell is an excellent way to expedite that process. If shampoo doesn't work to get the dye out, use lemon juice.
Dyeing your hair can be a messy process, so don't wear clothes you don't want ruined and consider doing it outside, so you don't get green dye all over your bathroom.