Put on a pair of plastic gloves to protect your hands from getting stained. You don't want to look like you've been dyeing Easter eggs.
Mix together one quarter cup of shampoo with six drops of food coloring in an empty shampoo bottle. (see Reference 1)
Close the shampoo bottle tightly and shake it thoroughly.
Place an old towel around your neck and a little petroleum jelly around your hair line to protect your face and clothes from stains.
Wet your hair. Lean over into your kitchen sink or tub and saturate your hair with water. Don't do this in the shower unless you want your entire body tinted with your chosen color.
Massage the shampoo/food coloring mixture into your hair. Apply it to all of your hair evenly.
Cover your hair with a shower cap and leave the mixture in for 30 minutes.
Rinse your hair thoroughly -- until the water runs clear -- and dry it.
Do a strand test to see how much food coloring you need to achieve the color you want.
Lighter hair will soak up color faster than darker hair, so leave the mixture on your head for longer than 30 minutes to get a darker color.
Food coloring stains anything it touches. Immediately wash off any drips that fall on your tub, sink or floor.