The pads have a thin layer of liquid crystals under a layer of plastic. Liquid crystals are materials that are on the threshold of conditions between liquid and solid. Different liquid crystals react to different stimuli, but these are temperature sensitive.
The color of a material depends on the way the material's molecules respond to light of different wavelengths (colors) and that in turn depends on the structure of the molecules. A piece of red paper has pigments in it whose molecules resonate with (or hum along at) and reflect the frequency of red light and it absorbs the other colors of the spectrum.
The liquid crystals in the pads (and the pigments in the pencils we distribute) are made of molecules whose properties change with temperature in such a way that it changes the way the molecules interact with colored light. When the pads are room temperature, they absorb all the light, and appear black. As they warm to body temperature, the size and shape of the crystals change, and they run through the color series resulting in the beautiful colors.
Light and color have fascinated and confused people for a long time. I have only scratched the surface of this subject here. It is a good one and your question is a good question. Thanks for asking.
