invisible marks that fluoresce under UV light

These invisible marks have been used for many different purposes: transforming art, transforming posters, transforming tattoos, escape room clues, rubber stamps on hands for secure entry,

paint markers

• Sakura Pen-Touch paint markers — this works fairly well

paint

• Rosco sells a fluorescent paint called "Invisible Blue", that's mostly invisible when painted on a light surface
• There are additives that you can mix into any paint. If you have matching paint, then the overlaying paint should match the underlying paint very well.
  • laundry detergent, which has a whiting agent [2] [3]
    • Wikipedia — Optical brightener