This is because the acid contained in both white vinegar and food dye combine with the hydrogen in water. A solution of water and food dye does not provide an appropriate acidic environment. Water is neutral with a pH of seven. However, white vinegar contains about 3 percent acetic acid, making it the perfect acidic environment. When egg shells are emerged in this acidic solution, calcium carbonate found in the eggshell reacts to the acid and produces carbon dioxide. This chemical reaction makes the food color pigment adhere to the shell, resulting in deeper and brighter color.
Dyeing eggs is a bit of a science experiment. Cooks can play around with the amount of vinegar and food color to get the desired color vibrancy.
The practice of decorating eggs began long ago, predating Christianity. The first eggs used were ostrich eggs, not chicken eggs. Instead of dye, the shell was embellished with gold and silver. These eggs where then placed in the graves of ancient Egyptians. In the Ukraine, coloring eggshells is called pysanky. This tradition has been practiced for thousands of years.Learn more about Dyeing