Thermochromism is the ability of a substance to change color due to changes in temperature. Mood rings are an example of an app that works using this effect.

The two thermochromic mechanisms are based on liquid crystal and dyes.

Liquid crystal is used when a certain precision is required, since its result can be manipulated for specific temperatures, but its color range for general applications is limited. Dyes allow a greater variety of colors to be obtained, but their thermal response is not as precise.