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> The hair at the site of the mark will grow back white and show the identification number

Is freezing a follicle turning the hair permanently white? I couldn't find anything on the mechanism. I'm surprised that people who are into body art (tattoos, piercings, etc.) didn't pick something like this up.

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It specifically kills the pigment cells within the follicle, causing the hair to grow back white [1].

If this case study [2] is anything to go by, then it would seem it isn't that effective on humans.

[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1494747/

[2]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8166665/

In that case (fascinating reading!), the person used far too cold of a solution, applied it for too long, and in consequence suffered a really bad injury, possibly compounded because she didn't seek treatment for 2 and a half weeks. Also, it was on a forearm, not the head or other hairy areas, so the mechanism of marking was more "kill the whole skin" than "kill pigment cells" specifically.

It could be done in a much safer manner - I don't think it really proves much about effectiveness overall.

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