Chromogenic materials are substances that change color in response to external stimuli, such as temperature, light, or chemical reactions. These materials can be found in various applications, including smart windows, which adjust their tint based on sunlight, and mood rings, which change color with body temperature. The color change in chromogenic materials is often reversible, meaning they can return to their original state once the stimulus is removed. This property makes them useful in fields like architecture, fashion, and electronics, where dynamic visual effects or energy efficiency are desired.