It all starts with red clay dirt from Roswell, NM, which is then hauled to the shirt maker's facility in Fort Worth, TX. There, the clay is mixed with water to form a thick, red-brown mud. When the mud is just right, 100% cotton Hanes or Fruit of the Loom white T-shirts are added to the mixture, and the transformation begins. Unless the mud is worked into the fabric, the shirts merely stain, so dyers coax the mud into the fabric of each shirt by hand. It takes a lot of time and effort to achieve a rich red-brown color.
So...what happens when you wash it? Because the shirts are dyed in red clay, using no artificial mordants or chemical fixatives, the iron oxide found naturally in clay is the dying agent. As a result, the color will bleed initially and fade slightly, which is why pre-washing separately in cold water before wearing is recommended. Because the mud is actually worked deep into the fabric, the shirts will never lose their color completely; but will instead, over time, fade to a pleasant buckskin color.