Thermochromic materials change colour as the temperature changes. These are used on contact thermometers made from plastic strips and test strips on the side of batteries (where the heat comes from a resistor under the thermochromic film). They are also used as food packaging materials that show you when the product they contain is cooked to the right temperature.
Photochromic materials change colour according to different lighting conditions. They are used for security markers that can only be seen in ultraviolet light.
