If you are of a ~certain age~, you probably remember Hypercolor.
It was a super cool clothing line all the teens wore that changed color based on your body temperature and the temperature of the air.
TOO COOL AM I RIGHT?
WHAT GREAT WONDER IS THIS?
TOO COOL AM I RIGHT?
Fire uses the same thermochromic ink used in Hypercolor shirts, but made safe to be used on the skin and hair. The product even changes color if you blush.
Fingers crossed!