Hypercolor

They contained a thermochromic pigment made by Matsui Shikiso Chemical of Japan, that changed between two colors—one when cold, one when warm.

[2][3] The effect could easily be permanently damaged, particularly when the clothing was washed in hotter than recommended water, ironed, bleached, or tumble-dried.

There are two common types of thermochromes: liquid crystals (used in mood rings) and leuco dyes (used in Hypercolor T-shirts).

Droplets of the thermochromic dye mixture are enclosed in transparent microcapsules, a few micrometers in diameter, bound to the fibers of the fabric.

This reaction increases the pH, which leads to closing of the lactone ring of the dye to convert it to its colorless (leuco) form.

Example of a Hypercolor T-shirt. A hair dryer was used to change the blue to turquoise .
Another example of a Hypercolor T-shirt.
Transformation between leuco and colored form of crystal violet lactone in response to varying acidity, which is caused by changing temperature