Generic Name: Phase Change Devices
They enable people to obtain reasonably precise (typically 1% of temperature) measurement of the surface temperature at a point where the paint or crayon mark or label has been placed.
Because of their irreversible nature (once changed in color or surface texture they do not revert back to their original condition when the temperature goes below their transformation temperature), they provide a record that the temperature at some time exceeded the temperature rating of the device.
That has some very special advantages in monitoring food and pharmaceutical storage and/or transport conditions.
The information page at About Temperature & Moisture Sensors , describes the technology in more detail and/or links to sites that do.