Usually, they are colourless in a dark place, and when sunlight is applied  the material changes and it becomes coloured. When the light source is removed the colour disappears. Examples might be tee-shirts. Different colour changes can take place when the light is brighter.
 

These change colour with changes in temperature.
The change in colour happens at a determined temperature, which can be varied.
They are used to make paints, inks or are mixed to moulding or casting materials for different applications. The cup above is plain, but when hot liquid is put into it, a picture appears on the outside.

These produce a brilliant light of different colours when stimulated by electricity. They can be used for making light stripes for decorating buildings, or for industrial and public vehicles safety precautions.

Shape-Memory Alloys are metals that, after being strained, at a certain temperature go back to their original shape.

a spring in a coffee pot can be 'trained' to release a valve at the right temperature. What is very important about these materials is, if you seriously damage them and then heat them up, they will go back to being undamaged.
 

A mobile refrigeration/heating box with 20-litre food and drink capacity. The low voltage required for its functioning (12 or 24 V) makes it suitable for auto vehicles, especially lorries. If the current passes one way it heats. If it passes the other it cools.

These are fantastic materials. They come as granules and when added to hot water they turn into a plastic that can be easily moulded as playdough can. When it cools it becomes hard like nylon. It can be reformed by heating up again. Very expensive but great for making prototypes and models