If you recall mood rings from the 1970s or Hypercolor t-shirts from the '90s, you probably won't be surprised that you can now color your hair with a dye that will change color in response to the temperature. The dye called FIRE from the company The Unseen debuted at Fashion Week.
According to Wired UK,
FIRE is designed to be responsive to temperature fluctuations, and is available in multiple colour ranges from bright red to subtle pastels. The data used to create the dye stems from the process of thermoregulation in the human skin and the colour change chemical reaction occurs in response to a certain stimuli - in this case, changes in the environment. When the temperature drops or rises, the carbon-based molecules at the core of the FIRE dye undergo a reversible reaction.
Then you'll have to watch out for this sort of reaction:
If they develop a dye that will go from white to cobalt blue, I'll be first in line. -via Uproxx