Clothing manufacturer Generra created these fad-ready T-shirts in the late '80s, but they really caught on in 1991. The shirts were dipped in temperature-sensitive pigment known as thermochromic leuco dyes. Heat changes caused the leuco dye to turn colorless temporarily, allowing the base color of the shirt to show through. For example, a yellow shirt dyed with blue leuco dye would appear green [source: ColorChange]. Heated areas of the shirt would therefore appear yellow, since the blue dye becomes colorless. That's why most hypercolor shirts reveal a lighter color rather than a darker one when heated.
For the most part, the heat-activated color change was brought about by the wearer's own body heat, resulting in a semi-random pattern on the shirt. A friend could easily leave a hypercolor handprint, as well. Unfortunately, excessive heat ruined the leuco dyes, so washing your hypercolor shirt in hot water would permanently undye it. This fragility no doubt contributed to the fad's quick fade – Generra went bankrupt in 1992.