Thermochromic pigment is a smart material which changes color in reaction to differences in heat. You will most likely to have encountered it as a novelty item often on mugs that rely on the most prevalent type which becomes transparent when heated revealing a message. The color change actually comes from the microscopic change in the material composition when heated that causes the crystal structure to realign. The accuracy varies between different products some are so accurate they can be used as thermometers others take boiling water to make changes happen.
Features
- Can be tuned to change colour at specfic heat ranges
- Can also go clear in some scenarios
- Functions for a great deal of time
- Can be included in most products
Constraints
- Extreme heat can destroy colour changing properties
- Some variations degrade faster
Used as thermometer it can give a constant up to date reading on a patient. 
Used for it’s aesthetic value thermochromic pigments can reveal a new style and give a visual reminder of heat usage
It can also serve as a warning, giving a visual cue to warn people that handles or other important items are no longer safe
For more info go to Open materials
Here’s an example of the colour change
