Just because you want to color your hair doesn't mean you want the same color all the time. Wouldn't it be nice if it could change with the weather, or whether or not you're inside? You might get your wish. The Unseen has developed a color-changing hair dye, Fire, that reacts to shifts in temperature -- it could be red outside and revert to a more natural color indoors. The carbon-based molecules in the dye alter their light absorption when they're subjected to temperature changes, producing different colors that you can reverse just by heading somewhere new.
Creator Lauren Bowker tells Wired that the dye is safe. It uses "less toxic" materials, such as irritants that are wrapped in polymers to minimize the damage to your hair and scalp. In theory, it shouldn't be any more harmful than the dye you buy at the store.
Fire still needs to be refined and fully assessed for safety before you can buy it. However, this isn't one of those far-off projects that will take many years to reach shelves -- there's already production-oriented testing underway. If everything goes smoothly, you could soon have a hair color evolves from moment to moment, not just whenever you feel up to a dyeing session.