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.