Hmm.. have to be quite visible. And contain enough ‘ink’ presumably to make sure it’s accurate. What a fantastic excuse to get a tattoo! I’ve never heard of this! I want one! A dog. no, a dragon. no, a bird… I like naked mole rats but they wouldn’t be that pretty – a dragonfly?
What colour do they turn from to, that might make a difference. You could have a traffic light. OR! A man who becomes naked when you are high!