The color change of Hypercolor shirts is based on combination of two colors: the color of the dyed fabric, which remained constant, and the color of the thermochromic dye. The dye is enclosed in microcapsules, tiny (few micrometers in diameter) drops of liquid sealed in a transparent shell, bound to the fibers of the fabric. The liquid is a leuco form of a dye (in this case crystal violet lactone), a weak acid (1,2,3-benzotriazole), and a quaternary ammonium salt of a fatty acid (myristylammonium oleate) dissolved in a solvent (1-dodecanol). At low temperatures, the weak acid forms a colored complex with the leuco dye, interrupting the lactone ring. At high temperatures, above 24-27 °C, the solvent melts and the salt dissociates, reversibly reacts with the weak acid and increases the pH. The pH change leads to closing of the lactone ring of the dye, which then regains its colorless (leuco) form.
Low temperatures allow a weak acid to react with the dye, converting it to its coloured form.
Whatever happened to those shirts anyhow? That they were easily ruined by washing them at higher than recommended temperatures didn't help, but I'm sure it had more to do with people not needing any more attention drawn to overactive armpits.