1/4 tsp food coloring
1 tbsp white vinegar
3/4 c hot water
Recipe
You will need one glass cup for each color dye that you wish to prepare. The cup should be approximately 3" in diameter and at least 3" deep. Any cup will do provided the egg is completely covered by the dye when placed into the cup.
For each color desired, mix food coloring, hot water, and white vinegar in cup. Carefully add one hard-boiled egg to each cup. Remove egg when desired color has been achieved. The longer the egg is in the coloring, the darker the color will become. Additional food coloring may be added to achieve darker colors. Be sure to remove the egg before adding any additional coloring.