It has been a remarkable transformation for an industry that was once a joke. When it was introduced in 1998, Mystic Tan, which is widely credited as the first mainstream spray tan, often resulted in a bathed-in-­Cheetos look. Yet in recent years, spray tan manufacturers perfected their recipes, decreasing the amount of ­dihydroxyacetone—the ingredient that darkens the skin—from 25 percent to just under 10 percent. Now spray tan “artists” have gone from maligned outsiders to wealthy specialists.