I would proudly support the “purple finger” idea. Of course, it would take a strong formula that was impossible to wash off within 24 hours or so, but I think that is doable.
Naturally, the Democrats would totally freak if it was tried. They view voter fraud as a natural right.
That's why they love "early voting".
With early voting, someone can vote over and over and over and over without losing out on siesta or midnight basketball.