Process. Thermocromic.

Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, e.g. in baby bottles (changes to a different color when cool enough to drink) or kettles (changes when water is at or near boiling point). Thermochromism is one of several types of chromism.

The two common approaches are based on liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.

Examples in Design:

Reflections:

Interesting process which hasn't been fully utilised in design perhaps. I'm interesting in it as a way to express a concept more effectively, for example the top image where a book called 'metamorphosis' does in fact morph. It needs to express something.