Sometimes seeing the stains of Electoral Ink made us surprise.....it surprised me as well, it made me think of the science behind it.
And what I found is that this ink formulation is developed by scientists at CSIR NPL and ink is manufactured by Mysore Varnisher and Paints for elections within India and 30 other countries.
The ink has some undisclosed percentage of silver nitrate solution( less than 25%) which reacts with proteins in our skin to form a complex which is insoluble in any solvent. It also contains a photosensitive dye which give voilet color on exposure of UV light. It only vanishes when your old skin cells die and new cells substitute in it's place .