Del Sol

Del Sol says some bats can see the colors on its shirts before you can. The Sandy company's Spectrachrome dyes stay invisible to people until ultraviolet light hits them, and its own explainer notes that certain species of bats can see the color while it is still switched off. Everyone else has to step outside.

The idea dates to the early 1990s, when Robert Pedersen, known as Captain Bob, and his son-in-law Steve Schone began searching, as the company puts it, for a way to bottle their idea about making a little sunshine go a long way. In 1994 they produced what the company calls the world's first T-shirt that changes color in sunlight. The print was designed to switch on cue outdoors and fade back to its original color indoors.

The first sales came from a small cart at Fashion Place Mall in Murray. A product that needs sun soon went looking for more of it, and in 1997 Del Sol opened its first full store in St. Thomas, in the U.S. Virgin Islands. The stores now sit in cruise ports and resort towns across the Caribbean, Mexico, Hawaii and Alaska.

The chemistry is the company's to explain. Del Sol says the dye does not actually change color. Instead, sunlight shifts the molecule into a shape the human eye can see, which the company's chemists call a molecular excitation transition. Del Sol says it is the only producer of Spectrachrome dyes, and that its chemists have tested more than 300,000 color changing variations since 1994.

The company also says the formula grew from color change technology NASA explored for the lenses on astronauts' helmets. That claim is Del Sol's alone, and the company handles it with a wink: "It's not rocket science, but then again, maybe it is."

The guarantee is easier to check. If a Del Sol item stops changing color in sunlight, the company replaces it for life. Its color changing sunglasses go further, with a lifetime replacement guarantee even if a pair is lost or stolen, for a $19.99 processing fee.

Retail spread with the tourism map. By 2010, when it first sold through Canadian drugstores, Del Sol ran more than 128 stores in 21 countries, Chain Drug Review reported, and until then it had sold only through its own shops, mostly in the Caribbean. The lineup has since grown well past T-shirts, to nail polish and a pickleball set whose paddles change color.

The shirts sell best next to a beach. The company that makes them still answers its mail on 10200 South in Sandy, where the sun shows up reliably enough in July.