WWF - thermo graphics

research into how to go about using temperature sensitive substances to change one graphic into another, it seems this was mainly used on T-shirts...

Hypercolor was a line of clothing, mainly T-shirts and shorts, that changed color with heat. They were manufactured by Generra (now a division of Public Clothing Company) and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and outside the US as Global Hypercolor. They contained a thermochromic (temperature sensitive) pigment made by Matsui Shikiso Chemical of Japan, that changed between two colors–one when cold, one when warm. The shirts were produced with several color change choices beginning in 1988. [1] The effect could easily be permanently damaged, particularly when the clothing was washed in hotter than recommended water.

 (wikipedia)

Thermochromism is the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses such as baby bottles (changes to a different color when cool enough to drink) and kettles. Thermochromism is one of several types of chromism.
The two basic 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.

Paints

Thermochromic paint is a relatively recent development in the area of color-changing pigments. It involves the use of liquid crystal or leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or transparent, therefore changing the appearance of the mug.