Sounds similar to glow in the dark ink, where the sun actually charges the ink, and changes its color. Was the shirt you bought a white color t-shirt or dark colored, becuase it seems like that site I linked to above only sells light colored shirts. I wonder why that would matter.i bought one of those shirts in the bahamas. It was a Bob marley shirt that would turn rasta colours when it hit the sun. Heat didn't affect it, like hypercolour shirts which used thermochromatic dye. The parts that turn colour are rough after a few washings, the shirt will only turn red and yellow in the sun. BTW, its only the parts with the hard white material that turns colour, the shirt itself does not.