There is a clear watery ink used (absorbent pad and rubber stamp) to make a mark
on the back of someone's hand to indicate "paid" admission at the swimming pool.
It's quite waterproof and hopefully non-toxic. It has no visible colour under
daylight or normal light, but shows up brilliantly under near UV. Perhaps it
could be added to a compatible solvent paint of any colour, to add UV
reflectance.
good luck