The egg whites are often dyed by submerging the cooked and peeled eggs for a couple of minutes in edible food coloring or food coloring gel mixed in water and vinegar (or sub with lemon juice). For a more natural approach, I suggest boiling the eggs in water with natural dye ingredients such as red cabbage, red wine, spinach, and carrot. Also, natural spirulina, or pitaya powders give a bright color and are all made with organic ingredients.