1. Mix water and blue food color in a large bowl.
2. Crack the shells of the hard boiled eggs gently with a spoon. Do not remove the shell.
3. Put the cracked eggs in the water and food coloring mixture and refrigerate for at least 2 hours.
4. Remove eggs from the refrigerator, peel carefully, and cut eggs in half lengthwise.
5. Scoop out the egg yolks and transfer to a bowl.
6. Set the whites aside.
7. Mash the yolks with the mayonnaise, green food coloring, mustard, and vinegar and pipe the yolk mixture into the egg whites.
8. Garnish with black sesame seeds.